py/obj: Convert make_new into a mp_obj_type_t slot.

Instead of being an explicit field, it's now a slot like all the other
methods.

This is a marginal code size improvement because most types have a make_new
(100/138 on PYBV11), however it improves consistency in how types are
declared, removing the special case for make_new.

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

py/objgetitemiter.c

@@ -60,7 +60,6 @@ STATIC MP_DEFINE_CONST_OBJ_TYPE(
     mp_type_it,
     MP_QSTR_iterator,
     MP_TYPE_FLAG_ITER_IS_ITERNEXT,
-    MP_TYPE_NULL_MAKE_NEW,
     iter, it_iternext
     );