as_deserialize_into_world(): provide a way to read a deserialized entities' IDs

I'm implementing prefabs, and I feel like `as_deserialize()` would result in at least one excess copy of the new instance.
`as_deserialize_into_world()` looks just right, but currently I see no way to know - which exactly Entities were deserialized?
It would be nice to have a callback or to return a collection of ids.