- Call all finalizers properly, in a safe-to-do-so order.
- Make finalizers work together with weak reference callbacks - namely,
  both should be able to get called, and weakref callbacks should be
  called before finalizers.
- Old style classes are also supported, but we always do an attribute
  lookup for tp_del because there's no tp_del slot. Since it would be
  too slow to do an attribute lookup all the time in GC, we just assume
  all old-style classes have an ordered finalizer.