Known issues:
The string.Formatter class, as discussed in the PEP, is incomplete.
Error handling needs to conform to the PEP.
Need to fix this warning that I introduced in Python/formatter_unicode.c:
Objects/stringlib/unicodedefs.h:26: warning: `STRINGLIB_CMP' defined but not used
Need to make sure sign formatting is correct, more tests needed.
Need to remove '()' sign formatting, left over from an earlier version of the PEP.
