The kstring integration in gix-attributes is unsound

gix-attributes (in state::ValueRef) unsafely creates a &str from a &[u8] containing non-UTF8 data, with the justification that so long as nothing reads the &str and relies on it being UTF-8 in the &str, there is no UB:

// SAFETY: our API makes accessing that value as `str` impossible, so illformed UTF8 is never exposed as such.

The problem is that the non-UTF8 str is exposed to outside code: first to the kstring crate itself, which requires UTF-8 in its documentation and may have UB as a consequence of this, but also to serde, where it propagates to e.g. serde_json, serde_yaml, etc., where the same problems occur.

This is not sound, and it could cause further UB down the line in these places that can view the &str.