Jython: Add full gc support to JyNI (and support ctypes)

by Stefan Richthofer for Python Software Foundation

JyNI is a compatibility layer with the goal to enable Jython to use native CPython extensions like NumPy. It already supports a fair part of Python's C-API and is for instance capable of running basic TKinter code. A main show-stopper for a production-release is its lack of garbage collection. I propose a concept how native gc can be emulated for JyNI in a way that is almost 100% compatible with CPython's native gc. With gc-support given, I assume support for ctypes to be straight forward.