refactor(eval): use arena when converting typvals to Object

Note: this contains two _temporary_ changes which can be reverted
once the Arena vs no-Arena distinction in API wrappers has been removed.
Both nlua_push_Object and object_to_vim_take_luaref() has been changed
to take the object argument as a pointer. This is not going to be
necessary once these are only used with arena (or not at all) allocated
Objects.

The object_to_vim() variant which leaves luaref untouched might need to
stay for a little longer.

src/nvim/main.c

@@ -1102,7 +1102,7 @@ static void command_line_scan(mparm_T *parmp)
           }
 
           Object md = DICTIONARY_OBJ(api_metadata());
-          msgpack_rpc_from_object(md, p);
+          msgpack_rpc_from_object(&md, p);
 
           msgpack_packer_free(p);
           const int ff_ret = file_flush(&fp);