Major re-work, TYPE_LOCK protects more things.

Rather than trying to update tp_base, tp_bases, and tp_mro while the
world is stopped, hold TYPE_LOCK when updating them.  This is much more
similar to how the base version of the code works and requires many
fewer changes.  Using stop-the-world turned out to be difficult to make
work correctly, mostly due to custom mro() methods but also for some
other reasons (for example, functions that potentially re-enter the
interpreter).

This re-work requires that we can call stop-the-world inside a critical
section and have the mutex still held after re-starting.  This seems to
work and I've added asserts to confirm it.  Stop-the-world is now only
used when updating either tp_flags or other type slots and not the three
type members listed above.

Author: nascheme