Issue #5988: Delete deprecated functions PyOS_ascii_formatd, PyOS_ascii_strtod, and PyOS_ascii_atof.

pystrtod.c, floatobject.c and dtoa.c.

PyOS_double_to_string, and deprecate PyOS_ascii_strtod and PyOS_ascii_atof.

Addresses the float -> string conversion, using David Gay's code which
was added in Mark Dickinson's checkin r71663.

Also addresses these, which are intertwined with the short repr
changes:

- Issue #5772: format(1e100, '<') produces '1e+100', not '1.0e+100'
- Issue #5515: 'n' formatting with commas no longer works poorly
    with leading zeros.
- PEP 378 Format Specifier for Thousands Separator: implemented
    for floats.

Add missing PyAPI_FUNC/PyAPI_DATA macros.