Add a Bump interner

[JonasAlaif]

Adds another interner option (sorry). Rather than holding interned Ts in individual Boxes (like the current Arena interner), this new Bump interner uses bump allocation. This approach is similar to how rustc handles interning.

My initial version is pretty bare-bones compared to the API of the other interners in this crate. I'd be happy to add functions as required.

An interesting thing to note is that one Bump could have multiple interner sets for different types while sharing the same arena for all of them. Unfortunately, I can't think of a way to make this general aside from defining by hand Bump2<T, U>, Bump3<T, U, V>, ...