Access world via entities
Comment style
Cloneability in ECS
Printable TerrainType
Timing utility
Migrating to entity component system
Mobs into play - massive spawn slowdown...
Disable in-place mutable components

Now all component changes must happen via set_component and be
trackable by systems.
Remove check_entity, was slowing things down a lot
Hash and Eq for entities
Spatial index module, things run fast again