So, @raskad and myself had a short discussion about the state of #736, and we came to the conclusion that it would be a good time to implement our own string interner; partly because the `string-interner` crate is a bit unmaintained (as shown by https://github.com/Robbepop/string-interner/pull/42 and https://github.com/Robbepop/string-interner/pull/47), and partly because it would be hard to experiment with custom optimizations for UTF-16 strings. I still want to thank @Robbepop for the original implementation though, because some parts of this design have been shamelessly stolen from it 😅.
Having said that, this PR is a complete reimplementation of the interner, but with some modifications to (hopefully!) make it a bit easier to experiment with UTF-16 strings, apply optimizations, and whatnot :)