catacomb-python.h, *.c: Move definitions back into implementation files.

The header was chock full of implementation details which weren't
needed by anything.  Move things which aren't needed further afield back
into their home files, and delete things like conversion functions which
aren't actually being used.

Also, some light reordering of the things that are left.

(This patch turns out to be remarkably commutative through the enormous
pile of changes coming up.)