Optimize slots: avoid temporary PyMethodObject

Issue #29507: Optimize slots calling Python methods. For Python methods, get
the unbound Python function and prepend arguments with self, rather than
calling the descriptor which creates a temporary PyMethodObject.

Add a new _PyObject_FastCall_Prepend() function used to call the unbound Python
method with self. It avoids the creation of a temporary tuple to pass
positional arguments.

Avoiding temporary PyMethodObject and avoiding temporary tuple makes Python
slots up to 1.46x faster. Microbenchmark on a __getitem__() method implemented
in Python:

Median +- std dev: 121 ns +- 5 ns -> 82.8 ns +- 1.0 ns: 1.46x faster (-31%)

Co-Authored-by: INADA Naoki <songofacandy@gmail.com>

Include/abstract.h

@@ -257,6 +257,12 @@ PyAPI_FUNC(PyObject *) _PyObject_Call_Prepend(
     PyObject *args,
     PyObject *kwargs);
 
+PyAPI_FUNC(PyObject *) _PyObject_FastCall_Prepend(
+    PyObject *callable,
+    PyObject *obj,
+    PyObject **args,
+    Py_ssize_t nargs);
+
 PyAPI_FUNC(PyObject *) _Py_CheckFunctionResult(PyObject *callable,
                                                PyObject *result,
                                                const char *where);