Add solution for serializing/deserializing World as part of a larger type

At the moment, the only way to deserialize a `World` is through `Registry::as_deserialize(_into_world)`. There are no types that implement `Deserialize`, which means it's not possible to include a world in a larger struct and implement `Deserialize` on that. It's possible with a custom `#[serde(deserialize_with = "path")]` on the struct, but that just shifts the problem.

I realise that part of the issue is how to pass the target `World` in, since Serde doesn't allow you to pass information in from the outside. That could perhaps be solved using the `scoped-tls-hkt` crate, or a custom solution like what is used for `Entity` deserialization.