Reproducible entity identifiers

Currently, we use GUIDs to link entities and their animations. This works, but is not ideal; my most concern is that this makes the game state non-repeatable (since GUIDs are created by random).

I'd prefer us to have some sequential identifiers for all entity kinds (perhaps strongly typed even) — and have it reproducible, so let's put a sequential generator (or several) into the game engine.

When implementing, look for the number of this issue in the project code base, and make sure to eliminate every mention.