segfault the interpreter during weakref clean up. Now any new weakrefs created
after __del__ is run are removed silently.
Fixes bug #1377858 and the weakref_in_del crasher for new-style classes.
Classic classes are still affected.
bugs which cause the interpreter to crash. I'm sure we can find a few
more. Many missing bugs deal with variations on unchecked infinite recursion
(like coerce.py).