Added support for an _object_size method in objects.
If defined the return value from that function will be added to the
size calculated by _object_size.
This is mainly useful to get more correct debug information about
memory usage, as an example when using Debug.pp_object_usage.
Callbacks have currently only been added to Image.Image and
Currently the normal memory_usge() function does not include these
sizes, it should probably do so, however.