UTF-8/16 transcoder

[Entomy]

Currently there are UTF-8 and UTF-16 encoder/decoder methods. These work by converting to and from Rune (UTF-32). While it's possible to do this through a round-trip, such as UTF-8 → UTF-32 → UTF-16, it should be obvious that transcoder methods would be faster than decoding and then encoding.

[Entomy]

I'm removing good first issue and making this difficult because, as it turns out, there's basically no direct recoders. Everyone's been doing the round trip decode-then-encode.

[Entomy]

This is going to wind up requiring a good knowledge of mathematics, then, since a new algorithm will need to be developed to do this.

[Entomy]

This should be called a transcoder. I've updated the title to reflect this.