Add an `enumerate` method to the Id vectors

[sonmarcho]

Or rename iter_indexed to enumerate (enumerate seems to be a more standard name for such method).

[Nadrieril]

I like "iter_enumerated", that's what https://doc.rust-lang.org/nightly/nightly-rustc/rustc_index/struct.IndexVec.html uses

[sonmarcho]

Sounds good