so object's can be resurrected even with a non-resurrecting
tp_dealloc.  The only thing that can happen at that point is
that the gc_visit function will get called.

This shows up in the new sqlalchemy tests: they use a subclass
of dict (defaultdict), which defines a custom tp_dealloc.  We call that,
which ends up calling dict_cls->tp_dealloc, which calls ~DenseMap
on the internal DenseMap.  The GC will accidentally keep the object
alive (via a stack reference or something similar), and call its
visit function on the next GC.  But ~DenseMap does not leave the map
in a consistent state, so iterating over it will end up producing
garbage values.  To solve this, add a new function that does
all the same memory-freeing, but does a tiny bit of extra work to keep
the DenseMap in a valid state for future gc_visit calls.