Add GC infrastructure: tracking bits, tp_clear by youknowone · Pull Request #6977 · RustPython/RustPython

GC bit operations (_PyObject_GC_TRACK/UNTRACK equivalent):
- Add set_gc_bit() helper for atomic GC bit manipulation
- Add set_gc_tracked() / clear_gc_tracked() methods
- Update is_gc_tracked() to use GcBits::TRACKED flag
- Call set_gc_tracked() in track_object()
- Call clear_gc_tracked() for static types (they are immortal)

tp_clear infrastructure (for breaking reference cycles):
- Add try_clear_obj() function to call payload's try_clear
- Add clear field to PyObjVTable
- Add clear() method to PyType's Traverse impl