ref(pii): Use external utf16string crate

[flub]

The relay-wstring crate has been split off to the utf16string crate,
this switches our code over.

Unsurprisingly straightforward. Caveat is that this is not 100% the same code backing this: the external crate uses the ByteOrder trait to read and write u16s instead of using u16::from_le_bytes() and u16::to_le_bytes(). Because of that there's some more panicking code in there, but the perf impact of that seemed very small in microbenchmarks. And the whole thing has seen less review. Seems to work fine though.

#skip-changelog

[jan-auer]

It would be nice to expose typedefs for this as typing out WStr<LittleEndian> every time is a little cumbersome.

[flub]

The shorthand is WStr<LE> which is also exposed as utf16string::LE. For signatures and struct definitions I choose the longer spelling as this isn't that common around here and makes reading easier when you're not used to it (i.e. myself in a year's time)

[jan-auer]

Makes sense, being explicit here helps for sure. I was thinking of utf16string::{WStrL, WStrB} or something similar to avoid the angle brackets and additional imports (similar to how io::Result is a shorthand for Result<io::Error>), but in the end it doesn't really matter.