Implement `Display` derive as a complement to `FromStr`

[abonander]

This exists in strum and derive-more but the former is complicated and the latter doesn't have FromStr or TryFrom<{repr}> derives for enums. I rather like the simplicity and narrowed scope of this crate and it'd be nice not to have to import multiple crates to get all the derives I need. I'd be glad to open a PR if you'd accept it.