Provide a trait for char slices

[owickstrom]

It seems the str trait and implementation are implemented in terms of folding an iterator of chars. Would it be possible to also expose a trait/impl for calculating the width of &[char]? That's the representation I have and a simple sum over char widths doesn't seem correct, AFAICT.

[Manishearth]

A more generic thing was proposed in #84 (comment)

The tricky thing is that the current algorithm works backwards, so it can be confusing for an iterator API to require a reversed iterator.

I think it would be fine to impl UnicodeWidthStr on the char array.

[owickstrom]

I see. Do you accept PRs? I could give it a go, even though I'm not exactly a Unicode expert.

[Manishearth]

Yes

[Manishearth]

lookup::str_width already takes an iterator.

[owickstrom]

@Manishearth thanks for the response! It turned out that I didn't need this in the end (the API I'm using already had the width information), so feel free to close this issue if you'd like.