Implementation of PEP 3101, Advanced String Formatting.

Known issues:

The string.Formatter class, as discussed in the PEP, is incomplete.

Error handling needs to conform to the PEP.

Need to fix this warning that I introduced in Python/formatter_unicode.c:
Objects/stringlib/unicodedefs.h:26: warning: `STRINGLIB_CMP' defined but not used

Need to make sure sign formatting is correct, more tests needed.

Need to remove '()' sign formatting, left over from an earlier version of the PEP.

Include/unicodeobject.h

@@ -1437,6 +1437,11 @@ PyAPI_FUNC(Py_UNICODE*) Py_UNICODE_strchr(
     const Py_UNICODE *s, Py_UNICODE c
     );
 
+PyObject *
+_unicodeformatter_iterator(PyObject *str);
+PyObject *
+_unicodeformatter_lookup(PyObject *field_name, PyObject *args,
+                         PyObject *kwargs);
 
 #ifdef __cplusplus
 }