Create game state serializer #4

We need a simple serialization mechanism for the game state.

This won't include network safe serialization explicitly (yet), but should hopefully not create any messes to clean up later when adding netcode primitives.

The idea here is first unlock saving changes to an existing game.

The second idea here is to unlock snapshots for replay testing, which is further away.

It can at least be network order-stable.

Then, define "Definitions" data that is not part of Game State.

After that, it should be network-safe (topologically avoid state that is inherently non-serializable like engine primitives, and then avoid serializing anything that crosses the Definitions boundary)