futf is a library for flexible UTF-8, or UTF-8 fragments. I don't know. Check out the API documentation.
Anyway, it takes an index into a byte buffer and tells you things about the UTF-8 codepoint containing that byte. It can deal with incomplete codepoint prefixes / suffixes at the ends of a buffer, which is useful for incremental I/O. It can also handle UTF-16 surrogate code units encoded in the manner of CESU-8 or WTF-8.
This is a low-level helper for tendril that might be useful more generally.