Const width helper function

[lunyav]

Id be interested in a helper function (since traits cant have const fns atm) which can evaluate the width of a &str (and char) at compile time. I could even propose a PR for that if others think this is a nice idea. And maybe even as an optional feature.

[Manishearth]

If it would be rather straightforward I'm in favor.

[lunyav]

Ive looked into it and found out that we would need rust-lang/rust#63997 solved (for making a const binary_search implementation) or generally have const traits. I could create a PR for my current effort, making many internal fns const.