Serializing and deserializing asset handles

Asset handles can now be round-tripped successfully during serialization and deserialization. This is particularly important for world assets — the serialization format written through DynamicWorld::serialize, previously called scenes.

This wasn't a matter of just slapping on some derives, because handles aren't raw data: they're a pointer to the actual loaded asset. As a result, there was no clear way to either persist or reconstruct one. The new HandleSerializeProcessor and HandleDeserializeProcessor solve this by storing a handle's identifying information (its asset path) on serialization, then reloading the asset from that path on deserialization. Pass them to TypedReflectSerializer::with_processor and TypedReflectDeserializer::with_processor if you need the same behavior in your own serialization pipelines.

Caveat

For this to work, your assets need to be correctly reflected. If your asset looks like:

#[derive(Asset, TypePath)]
struct MyAsset {
    ...
}

Change it to:

#[derive(Asset, Reflect)]
#[reflect(Asset)]
struct MyAsset {
    ...
}