We claim: 1. A miniature cryosurgical system, comprising: a compressor for pressurizing a gas mixture; a flexible, dual lumen, coaxial catheter, said catheter comprising: a hollow elongated inner supply tube, said supply tube having a proximal end connected in fluid flow communication with an outlet of said compressor; a hollow elongated outer return tube, said return tube having a proximal end connected in fluid flow communication with an inlet of said compressor; a heat exchanger mounted in fluid flow communication with said compressor and said catheter, said heat exchanger comprising: a high pressure passageway, said high pressure passageway having an inlet port connected in fluid flow communication with said outlet of said compressor, said high pressure passageway having a distal outlet port; a low pressure passageway, said low pressure passageway lying substantially alongside said high pressure passageway, said low pressure passageway having a distal inlet port, said low pressure passageway having an outlet port connected in fluid flow communication with said inlet of said compressor; and a substantially annular insulating jacket integrally formed within said heat exchanger by formation of substantially annular channels positioned radially outwardly from said high pressure passageway and said low pressure passageway, said annular insulating jacket being evacuated; wherein the direction of flow from said inlet port to said outlet port of said high pressure passageway is substantially opposite to the direction of flow from said inlet port to said outlet port of said low pressure passageway; an expansion element mounted within said distal end of said return tube, an inlet of said expansion element being connected in fluid flow communication with said distal outlet port of said high pressure passageway of said heat exchanger, an outlet of said expansion element being connected in fluid flow communication with said at least one distal inlet port of said low pressure passageway of said heat exchanger; and a heat transfer body mounted to said distal end of said return tube adjacent said expansion element, said heat transfer body having an inner surface exposed to said outlet of said expansion element and an outer surface exposed to ambient, for transferring heat from said outer surface of said heat transfer body to said inner surface of said heat transfer body. 2. A miniature cryosurgical system, comprising: a compressor for pressurizing a gas mixture; a flexible, dual lumen, coaxial catheter, said catheter comprising: a hollow elongated inner supply tube, said supply tube having a proximal end connected to an outlet of said compressor; a hollow elongated outer return tube, said return tube having a proximal end connected to an inlet of said compressor; a heat exchanger mounted within a distal end of said return tube, said heat exchanger comprising: a laminated assembly of a plurality of plates of heat conductive material; a high pressure passageway through said laminated assembly, said high pressure passageway having an inlet port connected to a distal end of said supply tube, said high pressure passageway having a distal outlet port; a low pressure passageway through said laminated assembly, said low pressure passageway lying substantially alongside said high pressure passageway, said low pressure passageway having a distal inlet port, said low pressure passageway having an outlet port in fluid flow communication with said return tube; and a substantially annular insulating jacket integrally formed within said heat exchanger by formation of substantially annular channels through said plates, said substantially annular channels being positioned radially outwardly from said high pressure passageway and said low pressure passageway, said annular insulating jacket being evacuated; wherein the direction of flow from said inlet port to said outlet port of said high pressure passageway is substantially opposite to the direction of flow from said inlet port to said outlet port of said low pressure passageway; and wherein each said passageway is arranged to create a tortuous flow path, to ensure turbulent flow of cryogenic gas therethrough; an expansion element mounted within said distal end of said return tube, an inlet of said expansion element being connected to said distal outlet port of said high pressure passageway of said heat exchanger, an outlet of said expansion element being in fluid flow communication with said at least one distal inlet port of said low pressure passageway; and a heat transfer body mounted to said distal end of said return tube adjacent said expansion element, said heat transfer body having an inner surface exposed to said outlet of said expansion element and an outer surface exposed to ambient, for transferring heat from said outer surface of said heat transfer body to said inner surface of said heat transfer body. 3. A miniature cryosurgical system, comprising: a compressor for pressurizing a gas mixture; a flexible, dual lumen, coaxial catheter, said catheter comprising: a hollow elongated inner supply tube, said supply tube having a proximal end connected to an outlet of said compressor; a hollow elongated outer return tube, said return tube having a proximal end connected to an inlet of said compressor; a heat exchanger mounted within a distal end of said return tube, said heat exchanger comprising: a plurality of plates of heat conductive material laminated in a stack along a longitudinal axis to form an elongate laminated assembly; a high pressure passageway through said laminated assembly, said high pressure passageway comprising a first plurality of holes through each said plate, said high pressure passageway having an inlet port connected to a distal end of said supply tube, said high pressure passageway having a distal outlet port; a low pressure passageway through said laminated assembly, said low pressure passageway lying substantially alongside said high pressure passageway, said low pressure passageway comprising a second plurality of holes through each said plate, said low pressure passageway having a distal inlet port, said low pressure passageway having an outlet port in fluid flow communication with said return tube; and a plurality of spacers laminated alternatingly between said plates to form a high pressure flow chamber and a low pressure flow chamber between each pair of adjacent said plates, wherein each said spacer includes a partition dividing said high pressure flow chamber from said low pressure flow chamber; wherein the direction of flow from said inlet port to said outlet port of said high pressure passageway is substantially opposite to the direction of flow from said inlet port to said outlet port of said low pressure passageway; and wherein each said passageway is arranged to create a tortuous flow path, to ensure turbulent flow of cryogenic gas therethrough; an expansion element mounted within said distal end of said return tube, an inlet of said expansion element being connected to said distal outlet port of said high pressure passageway of said heat exchanger, an outlet of said expansion element being in fluid flow communication with said at least one distal inlet port of said low pressure passageway; and a heat transfer body mounted to said distal end of said return tube adjacent said expansion element, said heat transfer body having an inner surface exposed to said outlet of said expansion element and an outer surface exposed to ambient, for transferring heat from said outer surface of said heat transfer body to said inner surface of said heat transfer body; wherein said partitions on said spacers are located to transversely offset adjacent said high pressure flow chambers from each other, and to transversely offset adjacent said low pressure flow chambers from each other, to direct substantial portions of each said passageway substantially transverse to said longitudinal axis of said heat exchanger. 4. A miniature cryosurgical system, comprising: a compressor for pressurizing a gas mixture; a flexible, dual lumen, coaxial catheter, said catheter comprising: a hollow elongated inner supply tube, said supply tube having a proximal end connected to an outlet of said compressor; a hollow elongated outer return tube, said return tube having a proximal end connected to an inlet of said compressor; a heat exchanger mounted within a distal end of said return tube, said heat exchanger comprising: an axially stacked assembly of laminated flat plates and flat spacers, said plates being alternated with said spacers; a first plurality of holes formed through said plates, forming a passageway for turbulent flow of high pressure cryogenic gas, each said high pressure hole being transversely offset from a corresponding high pressure hole in each adjacent said plate; a second plurality of holes formed through said plates, forming a passageway for turbulent flow of low pressure cryogenic gas, each said low pressure hole being transversely offset from a corresponding low pressure hole in each adjacent said plate; a substantially annular channel formed through each said plate and each said spacer to integrally form a substantially annular insulating jacket within said laminated assembly, said substantially annular channel being positioned radially outwardly from said high pressure and low pressure holes, said annular insulating jacket being evacuated; a plurality of spacers laminated to a first end of said stacked assembly, to create a manifold for connection of high pressure and low pressure gas lines to said heat exchanger; and an end plate laminated to a second end of said stacked assembly, said end plate having an orifice therethrough in flow communication with said high pressure holes; wherein the direction of flow from said inlet port to said outlet port of said high pressure passageway is substantially opposite to the direction of flow from said inlet port to said outlet port of said low pressure passageway; and a heat transfer body mounted to said distal end of said return tube adjacent said end plate, said heat transfer body having an inner surface exposed to said outlet of said orifice and an outer surface exposed to ambient, for transferring heat from said outer surface of said heat transfer body to said inner surface of said heat transfer body.