gh-125420: implement Sequence.__reversed__ API on memoryview objects

Objects/memoryobject.c

@@ -3310,6 +3310,8 @@ typedef struct {
 static PyObject *
 memoryiter_new(PyObject *self, int reversed)
 {
+    assert(reversed == 0 || reversed == 1);
+
     if (!PyMemoryView_Check(self)) {
         PyErr_BadInternalCall();
         return NULL;
@@ -3343,7 +3345,7 @@ memoryiter_new(PyObject *self, int reversed)
     it->it_index = reversed ? (it->it_length - 1) : 0;
     it->it_seq = _PyMemoryView_CAST(Py_NewRef(self));
     _PyObject_GC_TRACK(it);
-    return (PyObject *)it;
+    return _PyObject_CAST(it);
 }
 
 static void
@@ -3403,9 +3405,7 @@ PyTypeObject _PyMemoryIter_Type = {
     PyVarObject_HEAD_INIT(&PyType_Type, 0)
     .tp_name = "memory_iterator",
     .tp_basicsize = sizeof(memoryiterobject),
-    // methods
     .tp_dealloc = memoryiter_dealloc,
-    .tp_getattro = PyObject_GenericGetAttr,
     .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
     .tp_traverse = memoryiter_traverse,
     .tp_iter = PyObject_SelfIter,
@@ -3443,22 +3443,29 @@ static PyObject *
 memoryview___reversed___impl(PyMemoryViewObject *self)
 /*[clinic end generated code: output=25f9b4345f4720ad input=3c35e9267ad7fcc6]*/
 {
-    return memoryiter_new((PyObject *)self, 1);
+    // NOTE(picnixz): generic implementation via reversed(...) is faster
+    // if the iterator is not used or if zero or few items are iterated.
+    //
+    // When the number of items is sufficiently large (>= 2^5), benchmarks
+    // show that this implementation improves for-loop performances. Note
+    // that materializing the specialized reversed iterator is likely to
+    // be slower than materializing a generic reversed object instance.
+    return memoryiter_new(_PyObject_CAST(self), 1);
 }
 
 
 PyTypeObject _PyMemoryRevIter_Type = {
     PyVarObject_HEAD_INIT(&PyType_Type, 0)
-    "memory_reverseiterator",
-    sizeof(memoryiterobject),
+    .tp_name = "memory_reverseiterator",
+    .tp_basicsize = sizeof(memoryiterobject),
     .tp_dealloc = memoryiter_dealloc,
-    .tp_getattro = PyObject_GenericGetAttr,
     .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
     .tp_traverse = memoryiter_traverse,
     .tp_iter = PyObject_SelfIter,
     .tp_iternext = memorview_reverse_iternext,
 };
 
+
 PyTypeObject PyMemoryView_Type = {
     PyVarObject_HEAD_INIT(&PyType_Type, 0)
     "memoryview",                             /* tp_name */