* gc module internal structure and API
Add gc_state module with GcState, GcGeneration, GcDebugFlags, GcStats.
Replace gc module stubs with working API backed by gc_state.
Add gc_callbacks and gc_garbage to Context.
Add is_gc_tracked, gc_finalized, gc_get_referents to PyObject.
Collection is stubbed (returns 0) — actual algorithm to follow.
* fix dict/weakref/generators
* unmark test_asyncio
* apply review
Jeong, YunWon
714d1ce58b