[Feature Request] Add a BitSet join iterator

SPECS has a `BitSet` that can store a set of entities, and it can be used in component queries.

Here's an example from Amethyst, where the skin component stores a set of mesh entities:

https://github.com/amethyst/amethyst/blob/9453e966c468e9eef219b3c45a49a025a1dca2e8/amethyst_animation/src/skinning/systems.rs#L103

I'm not aware of any equivalent in legion. I think it would prove useful, at least for porting Amethyst to legion.