Add _PyObject_FastCall()

Issue #27128: Add _PyObject_FastCall(), a new calling convention avoiding a
temporary tuple to pass positional parameters in most cases, but create a
temporary tuple if needed (ex: for the tp_call slot).

The API is prepared to support keyword parameters, but the full implementation
will come later (_PyFunction_FastCall() doesn't support keyword parameters
yet).

Add also:

* _PyStack_AsTuple() helper function: convert a "stack" of parameters to
  a tuple.
* _PyCFunction_FastCall(): fast call implementation for C functions
* _PyFunction_FastCall(): fast call implementation for Python functions

Include/methodobject.h

@@ -37,6 +37,12 @@ PyAPI_FUNC(int) PyCFunction_GetFlags(PyObject *);
 #endif
 PyAPI_FUNC(PyObject *) PyCFunction_Call(PyObject *, PyObject *, PyObject *);
 
+#ifndef Py_LIMITED_API
+PyAPI_FUNC(PyObject *) _PyCFunction_FastCall(PyObject *func,
+    PyObject **args, int nargs,
+    PyObject *kwargs);
+#endif
+
 struct PyMethodDef {
     const char  *ml_name;   /* The name of the built-in function/method */
     PyCFunction ml_meth;    /* The C function that implements it */