Add unicode-segmentation

[florian-escola]

https://crates.io/crates/unicode-segmentation

I think this seems to be the go-to way for doing things like iterating over grapheme clusters.

Maybe some other crates from https://github.com/unicode-rs would make sense as well?

[nicoburns]

I'm definitely keen to add some unicode crates (and string handling in general). But I'm a little lacking in expertise on rust crates in this domain (having not really used these crates much myself) so I think some research will need to be done (either by me, or by someone else who can present their results) before I can confidently make recommendations.