Move _formatter_* methods from string type into _string module

Objects/stringlib/string_format.h

 /*
     string_format.h -- implementation of string.format().
 
     It uses the Objects/stringlib conventions, so that it can be
     compiled for both unicode and string objects.
 */
 
 
 /* Defines for Python 2.6 compatability */
 #if PY_VERSION_HEX < 0x03000000
@@ skipping 1162 matching lines @@
     (iternextfunc)formatteriter_next,   /* tp_iternext */
     formatteriter_methods,              /* tp_methods */
     0,
 };
 
 /* unicode_formatter_parser is used to implement
    string.Formatter.vformat.  it parses a string and returns tuples
    describing the parsed elements.  It's a wrapper around
    stringlib/string_format.h's MarkupIterator */
 static PyObject *
-formatter_parser(STRINGLIB_OBJECT *self)
+formatter_parser(PyObject *ignored, STRINGLIB_OBJECT *self)
 {
     formatteriterobject *it;
 
     it = PyObject_New(formatteriterobject, &PyFormatterIter_Type);
     if (it == NULL)
         return NULL;
 
     /* take ownership, give the object to the iterator */
     Py_INCREF(self);
     it->str = self;
@@ skipping 114 matching lines @@
     0};
 
 /* unicode_formatter_field_name_split is used to implement
    string.Formatter.vformat.  it takes an PEP 3101 "field name", and
    returns a tuple of (first, rest): "first", the part before the
    first '.' or '['; and "rest", an iterator for the rest of the field
    name.  it's a wrapper around stringlib/string_format.h's
    field_name_split.  The iterator it returns is a
    FieldNameIterator */
 static PyObject *
-formatter_field_name_split(STRINGLIB_OBJECT *self)
+formatter_field_name_split(PyObject *ignored, STRINGLIB_OBJECT *self)
 {
     SubString first;
     Py_ssize_t first_idx;
     fieldnameiterobject *it;
 
     PyObject *first_obj = NULL;
     PyObject *result = NULL;
 
     it = PyObject_New(fieldnameiterobject, &PyFieldNameIter_Type);
     if (it == NULL)
@@ skipping 21 matching lines @@
         goto done;
 
     /* return a tuple of values */
     result = PyTuple_Pack(2, first_obj, it);
 
 done:
     Py_XDECREF(it);
     Py_XDECREF(first_obj);
     return result;
 }