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>

extmod/modframebuf.c

@@ -833,7 +833,7 @@ STATIC MP_DEFINE_CONST_OBJ_TYPE(
     mp_type_framebuf,
     MP_QSTR_FrameBuffer,
     MP_TYPE_FLAG_NONE,
-    framebuf_make_new,
+    make_new, framebuf_make_new,
     buffer, framebuf_get_buffer,
     locals_dict, &framebuf_locals_dict
     );