Some rearranging of serialize/deserialize code

[Rua]

The following changes are made, some of which are breaking.

• Canon is moved out of Registry so that it's now a free-standing type. That way there's a clear separation of concerns, between handling Entity and handling component types.
• The with_entity_serializer methods on WorldSerializer and WorldDeserializer are consequently removed, while World::as_serializable and Registry::as_deserialize(_into_world) take an extra EntitySerializer argument.
• Canon now handles its locking internally, instead of having to be wrapped in a lock.
• CustomEntitySerializer's methods take &self, possible due to the change above.
• EntitySerializer now has a blanket implementation for anything that implements CustomEntitySerializer.
• The custom run_as_context logic is replaced with the scoped-tls-hkt crate, which does this more cleanly and is better-tested.
• The ENTITY_SERIALIZER variable, introduced by the change above, is made public and documented. This fixes Allow serializing/deserializing Entity outside World #228.

I think that the EntitySerializer and CustomEntitySerializer traits could do with better names, but I've held off on that as I can't think of names that really work well.

[FluffyCreature]

I think it would probably be better to hide the scoped-tls-hkt interface so there can be less dependency. Maybe consider to wrap ENTITY_SERIALIZER variable inside a function? (similar to the original run_as_context)

[TomGillen]

These changes look good to me, although I agree with @FluffyCreature that it would be better to hide the dependency from users (even though we don't follow this rule of thumb very consistently elsewhere).

[Rua]

I've made ENTITY_SERIALIZER private, and added a set_entity_serializer function as a simple wrapper around it.