Modified interface to _Py_[String|Unicode]InsertThousandsGrouping, in anticipation of fixing issue 3140.

 http://mail.python.org/pipermail/python-dev/2008-June/079988.html

Python 2.6 should stick with PyString_* in its codebase.  The PyBytes_* names
in the spirit of 3.0 are available via a #define only.  See the email thread.

Adds 'n' as a format specifier for integers, to mirror the same
specifier which is already available for floats.  'n' is the same as
'd', but inserts the current locale-specific thousands grouping.

I added this as a stringlib function, but it's only used by str type,
not unicode.  This is because of an implementation detail in
unicode.format(), which does its own str->unicode conversion.  But the
unicode version will be needed in 3.0, and it may be needed by other
code eventually in 2.6 (maybe decimal?), so I left it as a stringlib
implementation.  As long as the unicode version isn't instantiated,
there's no overhead for this.

Implemented grouping, with tests.
Cleaned up PyOS_ascii_formatd by breaking reformatting into smaller functions.

Deleted unused macro ISXDIGIT.

Added "Z" format_char to PyOS_ascii_formatd to support empty float presentation type.
Renamed buf_size in PyOS_ascii_formatd to more accurately reflect it's meaning.
Modified format.__float__ to use the new "Z" format as the default.
Added test cases.

Trim leading zeros from a floating point exponent, per C99.  See issue 1600.  As far as I know, this only affects Windows.  Add float type 'n' to PyOS_ascii_formatd (see PEP 3101 for 'n' description).

and adds errors for -0x.

and atof().