Note a curious extension to the std C rules:  x, X and o formatting can never produce
a sign character in C, so the '+' and ' ' flags are meaningless for them.  But
unbounded ints *can* produce a sign character under these conversions (no fixed-
width bitstring is wide enough to hold all negative values in 2's-comp form).  So
these flags become meaningful in Python when formatting a Python long which is too
big to fit in a C long.  This required shuffling around existing code, which hacked
x and X conversions to death when both the '#' and '0' flags were specified:  the
hacks weren't strong enough to deal with the simultaneous possibility of the ' ' or
'+' flags too, since signs were always meaningless before for x and X conversions.
Isomorphic shuffling was required in unicodeobject.c.
Also added dozens of non-trivial new unbounded-int test cases to test_format.py.