compute utf16 literal in const context

[Lokathor]

[thomcc]

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=d4bf36ff8d49186c4a299a81e67c1d01

Written a while ago for some apple code, but should be usable. It has some low hanging (compile-time) optimization fruit, but maybe i'll do that some other time.

[Lokathor]

ah excellent!

there's also this, which was PR'd to the utf16_lit crate, but that PR hasn't yet been published: https://github.com/Lokathor/utf16_lit/blob/main/src/lib.rs

I just need to take some time to do a write up of it all.