Function utf8string_fmt 

pub open spec fn utf8string_fmt() -> Mapped<Refined<Tail, PredFnSpec<Seq<u8>>>, FnSpecMapper<Seq<u8>, Seq<char>>>

{
    Mapped {
        inner: Refined(Tail, |bytes: Seq<u8>| vstd::utf8::valid_utf8(bytes)),
        mapper: (
            |bytes: Seq<u8>| vstd::utf8::decode_utf8(bytes),
            |chars: Seq<char>| vstd::utf8::encode_utf8(chars),
        ),
    }
}