For weakly-referenced objects that are garbage, we end up freeing their
attributes within that collection (assuming they are garbage as well).
This means that these objects are in a state that is not quite dead (we
don't want to allocate anything else in that space and clobber their
weakreflist), and not quite alive (their attributes point to garbage
memory).

So, change the handling to finish looking at those objects in the collection
that they become garbage, and then free them to make them properly dead.

weakrefs-handling is unavoidably reentrant, but now we only have to worry
about fully-dead (the referents) or fully-alive (the weakrefs) objects.