Allow iterating over audiences

[bittrance]

I suspect that like me, the most common operator on OneOrMany is to check whether it contains a particular audience value. That currently is more verbose than it needs to be.

This PR allows getting an "read-only" iterator from a OneOrMany. This means that:

let valid = match jwt.claims().aud {
    jwtk::OneOrMany::One(ref aud) if aud == "my-aud" => true,
    jwtk::OneOrMany::Vec(ref auds) if auds.iter().any(|a| a == "my-aud") => true,
    _ => false,
};

becomes:

let valid = jwt.claims().aud.iter().any(|a| a == "my-aud");

Which I think is a small but significant improvement.