[x] Modify pools and list to be more efficient
[x] Rework Entity:give to remove any previous component and re-add it.
[x] Add Entity:ensure (maybe?)
[ ] Put pools in the Instance and invert dependency.
[ ] Share pools between systems
[x] Remove System callbacks
[x] Put .added and .removed in pools so they can be iterated over
[x] Implement assemblages
[x] Do :apply automatically with marked entities
[x] Remove Entity.components. Use Entity[Component] instead

[ ] Add missing documentation
[ ] Fix current documentation
[ ] Write unit tests
[ ] Write examples