Emit METH_FASTCALL code in Argument Clinic

Issue #27810:

* Modify vgetargskeywordsfast() to work on a C array of PyObject* rather than
  working on a tuple directly.
* Add _PyArg_ParseStack()
* Argument Clinic now emits code using the new METH_FASTCALL calling convention

Include/modsupport.h

@@ -58,10 +58,13 @@ typedef struct _PyArg_Parser {
 } _PyArg_Parser;
 #ifdef PY_SSIZE_T_CLEAN
 #define _PyArg_ParseTupleAndKeywordsFast  _PyArg_ParseTupleAndKeywordsFast_SizeT
+#define _PyArg_ParseStack  _PyArg_ParseStack_SizeT
 #define _PyArg_VaParseTupleAndKeywordsFast  _PyArg_VaParseTupleAndKeywordsFast_SizeT
 #endif
 PyAPI_FUNC(int) _PyArg_ParseTupleAndKeywordsFast(PyObject *, PyObject *,
                                                  struct _PyArg_Parser *, ...);
+PyAPI_FUNC(int) _PyArg_ParseStack(PyObject **args, Py_ssize_t nargs, PyObject *kwnames,
+                                  struct _PyArg_Parser *, ...);
 PyAPI_FUNC(int) _PyArg_VaParseTupleAndKeywordsFast(PyObject *, PyObject *,
                                                    struct _PyArg_Parser *, va_list);
 void _PyArg_Fini(void);