Figure out how to get string display width

[yongrenjie]

Inside the pretty.ml module we're using String.length a lot, when we should really be using the display width of the string.

e.g.

# String.length "😄";;
- : int = 4

when it should only really be 2.

[yongrenjie]

This is the equivalent in Haskell: https://hackage.haskell.org/package/vty-5.38/docs/Graphics-Text-Width.html (https://github.com/jtdaugherty/vty)

It's a wrapper around a C library. Now, how do we do that in Ocaml...

• wcwidth (and wcswidth) are actually part of the POSIX spec: https://man7.org/linux/man-pages/man3/wcwidth.3.html
• There is in fact an OCaml library doing this: https://ocaml.org/p/charInfo_width/0.1.0 (https://github.com/kandu/charInfo_width) but unfortunately pulls in more dependencies (Camomile) than I would like