`Level` is not `Hash` – can I make it so?

[dvdplm]

I was trying to use Level as part of the key to a HashSet and realized that it is not Hash. Given that LevelInner is Hash I wonder if the outer type can be too.

Happy to open a PR.

[hawkw]

I think it would be fine to derive Hash for Level, please feel free to open a PR :)