Consider renaming entity::Allocator

[Anders429]

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`).