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>

ports/cc3200/misc/mpirq.c

@@ -194,7 +194,6 @@ MP_DEFINE_CONST_OBJ_TYPE(
     mp_irq_type,
     MP_QSTR_irq,
     MP_TYPE_FLAG_NONE,
-    MP_TYPE_NULL_MAKE_NEW,
     call, mp_irq_call,
     locals_dict, &mp_irq_locals_dict
     );