use val_temporal_packed(field_type()) internally.
This is not friendly to new data types, e.g. it won't work for a new implementation for TIMESTAMP (needed for MDEV-15654) which transfers TIMESTAMP data inside the server using my_time_t representation rather than MYSQL_TIME representation.
This task is to remove Item::val_temporal_packed() and split implementations of the mentioned methods into separate methods for TIME and DATETIME:
These methods will use val_time_packed() and val_datetime_packed() directly.