Edward Loper's cool and massive refactoring of doctest.py, merged from

the tim-doctest-merge-24a2 tag on the the tim-doctest-branch branch.
We did development on the branch in case it wouldn't land in time for
2.4a2, but the branch looked good:  Edward's tests passed there, ditto
Python's tests, and ditto the Zope3 tests.  Together, those hit doctest
heavily.