objsingleton: New home for Ellipsis and NotImplemented.

Having NotImplemented as MP_OBJ_SENTINEL turned out to be problematic
(it needs to be checked for in a lot of places, otherwise it'll crash
as would pass MP_OBJ_IS_OBJ()), so made a proper singleton value like
Ellipsis, both of them sharing the same type.

py/obj.c

@@ -62,14 +62,6 @@ void mp_obj_print_helper(const mp_print_t *print, mp_obj_t o_in, mp_print_kind_t
         return;
     }
 #endif
-
-    #if MICROPY_PY_BUILTINS_NOTIMPLEMENTED
-    if (o_in == MP_OBJ_SENTINEL) {
-        mp_printf(print, "%q", MP_QSTR_NotImplemented);
-        return;
-    }
-    #endif
-
     mp_obj_type_t *type = mp_obj_get_type(o_in);
     if (type->print != NULL) {
         type->print((mp_print_t*)print, o_in, kind);