Some nifty doctest extensions from Jim Fulton, currently used in Zope3.

Some nifty doctest extensions from Jim Fulton, currently used in Zope3.
I won't have time to write real docs, but spent a lot of time adding
comments to his code and fleshing out the exported functions' docstrings.
There's probably opportunity to consolidate how docstrings get extracted
too, and the new code for that is probably better than the old code for
that (which strained mightily to recover from 2.2's new class/type
gimmicks).