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/funcobject.h

@@ -58,6 +58,13 @@ PyAPI_FUNC(int) PyFunction_SetClosure(PyObject *, PyObject *);
 PyAPI_FUNC(PyObject *) PyFunction_GetAnnotations(PyObject *);
 PyAPI_FUNC(int) PyFunction_SetAnnotations(PyObject *, PyObject *);
 
+#ifndef Py_LIMITED_API
+PyAPI_FUNC(PyObject *) _PyFunction_FastCall(
+    PyObject *func,
+    PyObject **args, int nargs,
+    PyObject *kwargs);
+#endif
+
 /* Macros for direct access to these values. Type checks are *not*
    done, so use with care. */
 #define PyFunction_GET_CODE(func) \