feat(engine): Entity id versioning

[0ffz]

Entities can come and go quickly for plugins like Looty, where references to itemstacks are very unstable.

We may wish for one entity to reference another, and be sure that this reference corresponds to only that entity. Currently, if the reference entity gets removed, another will take its index (ID).

One solution to this is to reserve some bits of an entity ID to be tied to the version. There is a great page on this in the ECS back and forth blog: https://skypjack.github.io/2019-05-06-ecs-baf-part-3/. The implicit list idea is also pretty good, though performance wise really doesn't matter to us right now.

I'd like to essentially implement the exact system outlined in that blog, though how the syntax will look for end users needs to be decided.

We should also start thinking about how we'll be storing permanent entity references. Do we want to use UUIDs? What do we do when a reference is in unloaded chunks? There are enough questions there that I think it should be its own issue for later.