Argh.  I lost 1.5 day's worth of work on revision 6 of this patch last
night, due to me being tired and over-aggressively cleaning my working
directories.  I will have to reconstruct it from memory, hopefully
Tuesday.  (I basically know what I did, and going through the process
again should be much quicker.)

I have more C fixes by the way:

  * PyTypeObject needs to also expose __text_signature__.  Otherwise
    builtin classes can't have signatures.

  * There are a bunch of default docstrings for tp_ etc. slots
    (__add__, __call__) that live in typeobject.c.  I'll hand-convert
    them to have signatures and reasonable docstrings.