Add Index impls for IdHashMap and IdOrdMap

[ChrisJr404]

Hello! This picks up #9, adding Index impls for IdHashMap and IdOrdMap so you can look an item up with map[&key], the way std's HashMap and BTreeMap let you. Like those, indexing panics when the key is absent, and get is still there for the non-panicking version.

You mentioned trouble lining up the lifetimes, and I ran into the same wall at first. The issue is that get's query bound ties the key lifetime to the borrow of self, but Index::index doesn't let you name that borrow in the impl. Making the bound higher-ranked over the key lifetime (for<'k> Equivalent<T::Key<'k>>, and the Comparable equivalent for the ord map) gets it to compile and still covers the normal cases, like &str keys queried with str. There's a test for that borrowed-key case on both maps so it doesn't silently regress.

I kept this to the two single-key maps that the issue asks about. BiHashMap/TriHashMap have more than one key, so Index there is a bit more of a design question and I left it out.

Added tests and doc examples for both, plus a CHANGELOG entry.