Re-shuffle Obj::type_inst and BaseObjInst::type_inst

There was a moment during the refactor that I was thinking about getting
rid of the `__type__` attribute as the source of truth for the type
instance, but I think that was a bit more than I could chew. However I
forgot to re-add the default implementation for Obj::type_inst, so that
has been added back in.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/obj/macros.rs

@@ -23,10 +23,6 @@ macro_rules! impl_base_obj {
             self.$base_name.attrs_mut()
         }
 
-        fn type_inst(&self) -> ObjP {
-            self.$base_name.type_inst()
-        }
-
         fn as_any(&self) -> &dyn std::any::Any {
             self
         }