Moji: a small interface for generic rune handling in Zig

I have been working on a library called Bunsho and experimenting with ways to handle text parsing generically. Bunsho has an interface called Moji. The interface handles classification and transformation of individual runes, represented as u21. I usually use char to mean u8 and rune to mean u21, similar to Go. I think Zig adopting similar terminology could be useful, but that is a separate discussion. The idea behind Moji is to let text-processing code operate on runes without hard-coding one...