prefab handle + save file = undefined about sim-ecs HOT 1 OPEN

Tags landed in v0.3.0, which makes it easy to tag entities with the prefab handle. The logic should work like this:

1. When loading a prefab, generate a UUID as handle (instead of a number). Tag all new entities with the handle and use a way for the tag which makes it distinguishable as a prefab handle
2. When loading a save or prefab, fill the prefab-cache (world.entityLinks), too
3. Unloading will work out of the box

For now, storing and loading the actual handle is left to the user. I am open to ideas to make it simpler

from sim-ecs.