Add __del__ callbacks. They are too useful to leave out.

XXX Remaining problems:

- The GC module doesn't know about these; I think it has its reasons
  to disallow calling __del__, but for now, __del__ on new-style
  objects is called when the GC module discards an object, for better
  or for worse.

- The code to call a __del__ handler is really ridiculously
  complicated, due to all the different debug #ifdefs.  I've copied
  this from the similar code in classobject.c, so I'm pretty sure I
  did it right, but it's not pretty. :-(

- No tests yet.