Currently, we don't track instances of certain heap types based on the assumption that "no members" == "no reference cycles".

Unfortunately, it's still possible to create untracked reference cycles with one's parents. The following program leaks memory:

while True:
    class C:
        __slots__ = ()
    C.i = C()
    del C

The fix is simple: track all instances of user-defined classes, no exceptions. I'm not sure we were actually getting any real wins from the old behavior anyways.