Wrong encoding of Rust unicode chars

[maximebuyse]

Rust chars are unicode characters, while chars in OCaml, Rocq, F*, and more are just one byte. This means our handling of char literals is wrong in the OCaml engine AST (I fixed that in the private hax-evit, will be upstreamed here later), for all non-ASCII chars. And our encoding of chars in general in most backends is also wrong.

[maximebuyse]

fn main() {
    let _ = 'А';
}

Open this code snippet in the playground