`Index` trait should provide `get(&self, key: &Q) -> Option<&V>`

[solidsnack]

• Or a similar trait should exist and be provided by the standard indexable collections.

• Or maybe there is a generic way to do this with indexable collections already, in which case it would be nice to see it documented alongside Index.

• Or a Contains trait with contains(&self, key: &Q) -> bool can be used to similar effect.

[eddyb]

Also see #1679.

[solidsnack]

Will #1679 / rust-lang/rust#35729 apply for BTreeMap and friends? It is unclear to me at first reading since the discussion there is in terms of slices.