bring in the cpython implementation of weakrefs, and post init_weakref()
overwrite some fields in the BoxedClass's for weakref.ref/proxy/callableproxy
so that they participate in our GC (we also make use of their tp_traverse
functions for scanning and tp_clear functions as their simple_destructor.)

as we sweep the heap:

1) any unreachable objects that have weakreferences are kept alive
   and placed in an std::list called weakly_referenced.

2) any unreachable weakref.ref objects are cleared and removed from
   their referent's list.

After sweeping the entire heap, we then loop over the objects in
weakly_referenced.  If an object in the list still has weak references,
we loop over them clearing their target (setting it to None), and
calling their callback if they have one.  test/tests/weakref1.py tests this.