Issue #3158: doctest can now find doctests in functions and methods

Issue #3158: doctest can now find doctests in functions and methods
written in C.

As a part of this, a few doctests have been added to the builtins module
(on hex(), oct(), and bin()), a doctest has been fixed (hopefully on all
platforms) on float, and test_builtins now runs doctests in builtins.