Relax lifetimes returned by methods of Utf8StringRef

[DCNick3]

Previously, Utf8StringRef::as_bytes and some other methods returned references with lifetime bound to &self, while the actual reference lifetime is 'a.

With this change, a function life this could now be written:

fn extract_bytes<'a>(value: rmpv::ValueRef<'a>) -> &'a [u8] {
    match auth_request {
        rmpv::ValueRef::String(s) => s.as_bytes(),
        rmpv::ValueRef::Binary(s) => s,
        _ => panic!(),
    }
}