Consider renaming `entity::Allocator`

I'm not in love with the use of "Allocator" in this context. It conflicts with the other regular usage of the word "allocator" in relation to heap allocations, which I imagine would only be confusing for someone looking at this library's private API with fresh eyes.

The function of the `entity::Allocator` is to store the locations of entities (meaning their current archetype and index) using a generational index system. `allocate()` and `allocate_batch()` find a free location slot and register the entity's location there, while the other methods simply return the current location or modify that location.

So perhaps `entity::Index` or `entity::Map` would be better? Or `entity::LocationMap`? "Map" denotes key-value, and the current system is just a bunch of locations keyed by generational index (entity::Identifier` -> `Location`).