Fixes for shared 2.6 code that implements PEP 3101, advanced string

Fixes for shared 2.6 code that implements PEP 3101, advanced string
formatting.

Includes:
 - Modifying tests for basic types to use __format__ methods, instead
   of builtin "format".
 - Adding PyObject_Format.
 - General str/unicode cleanup discovered when backporting to 2.6.
 - Removing datetimemodule.c's time_format, since it was identical
   to date_format.

The files in Objects/stringlib that implement PEP 3101 (stringdefs.h,
unicodedefs.h, formatter.h, string_format.h) are identical in trunk
and py3k.  Any changes from here on should be made to trunk, and
changes will propogate to py3k).