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/eval.c

@@ -1355,7 +1355,7 @@ Object eval_foldtext(win_T *wp)
     retval = STRING_OBJ(NULL_STRING);
   } else {
     if (tv.v_type == VAR_LIST) {
-      retval = vim_to_object(&tv);
+      retval = vim_to_object(&tv, NULL, false);
     } else {
       retval = STRING_OBJ(cstr_to_string(tv_get_string(&tv)));
     }