py/obj: Merge getiter and iternext mp_obj_type_t slots.

The goal here is to remove a slot (making way to turn make_new into a slot)
as well as reduce code size by the ~40 references to mp_identity_getiter
and mp_stream_unbuffered_iter.

This introduces two new type flags:
- MP_TYPE_FLAG_ITER_IS_ITERNEXT: This means that the "iter" slot in the
  type is "iternext", and should use the identity getiter.
- MP_TYPE_FLAG_ITER_IS_CUSTOM: This means that the "iter" slot is a pointer
  to a mp_getiter_iternext_custom_t instance, which then defines both
  getiter and iternext.

And a third flag that is the OR of both, MP_TYPE_FLAG_ITER_IS_STREAM: This
means that the type should use the identity getiter, and
mp_stream_unbuffered_iter as iternext.

Finally, MP_TYPE_FLAG_ITER_IS_GETITER is defined as a no-op flag to give
the default case where "iter" is "getiter".

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

py/objnamedtuple.c

@@ -163,7 +163,7 @@ STATIC mp_obj_t mp_obj_new_namedtuple_type(qstr name, size_t n_fields, mp_obj_t
     MP_OBJ_TYPE_SET_SLOT(type, binary_op, mp_obj_tuple_binary_op, 2);
     MP_OBJ_TYPE_SET_SLOT(type, attr, namedtuple_attr, 3);
     MP_OBJ_TYPE_SET_SLOT(type, subscr, mp_obj_tuple_subscr, 4);
-    MP_OBJ_TYPE_SET_SLOT(type, getiter, mp_obj_tuple_getiter, 5);
+    MP_OBJ_TYPE_SET_SLOT(type, iter, mp_obj_tuple_getiter, 5);
     MP_OBJ_TYPE_SET_SLOT(type, parent, &mp_type_tuple, 6);
     return MP_OBJ_FROM_PTR(o);
 }