`Index` implementations for `IdOrdMap`/`IdHashMap`

[tbu-]

Would a PR adding them be accepted?

std's HashMap and BTreeMap can be indexed using []. It would be nice if IdOrdMap and IdHashMap provided the same functionality.

[sunshowers]

Hey! Yes a PR would be accepted, though I had trouble lining up the lifetimes when I gave it a shot. If it's infeasible to implement Index itself, it's okay to have a .index method.