Backport of r63826.

Backport of r63826.

Optimization of str.format() for cases with str, unicode, int, long,
and float arguments.  This gives about 30% speed improvement for the
simplest (but most common) cases.  This patch skips the __format__
dispatch, and also avoids creating an object to hold the format_spec.

Unfortunately there's a complication in 2.6 with int, long, and float
because they always expect str format_specs.  So in the unicode
version of this optimization, just check for unicode objects.  int,
float, long, and str can be added later, if needed.