Try to optimize `Key::encode`

It might be worthwhile to return something like `Cow` from [`Key::encode`](https://docs.rs/storey/latest/storey/containers/map/key/trait.Key.html#tymethod.encode). Some common cases (`String`?) don't need us to compute a value.

Start with Key::encode in the linked storey map key trait documentation and inspect how key values, especially String, are currently encoded. Determine which common cases could avoid computing a value and define the API and behavior needed for the optimization; done means the approach is validated without changing key encoding semantics.

Written by the indexing model from the issue text.