Backport of PEP 3101, Advanced String Formatting, from py3k.

Highlights:
 - Adding PyObject_Format.
 - Adding string.Format class.
 - Adding __format__ for str, unicode, int, long, float, datetime.
 - Adding builtin format.
 - Adding ''.format and u''.format.
 - str/unicode fixups for formatters.

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).

Include/formatter_string.h

@@ -0,0 +1,12 @@
+PyObject *
+string__format__(PyObject *self, PyObject *args);
+
+PyObject *
+string_long__format__(PyObject *self, PyObject *args);
+
+PyObject *
+string_int__format__(PyObject *self, PyObject *args);
+
+PyObject *
+string_float__format__(PyObject *self, PyObject *args);
+