There are still some things left to do:
* Implement despawn
* Maybe shrink SlotVector if there's a lot more free slots than occupied slots
* Implement iteration over entities

I know, I know, this segfaults on exit.

I need to make each mod subclass a Swan::Mod superclass
so that they can have EventListener structs which are
destroyed when the mod exits, and the mod has to exit
before the world goes away so that mods' EventListener
dtors can access their respective EventEmitters.