Make AttrValue's tokens() and atom() return or panic

[nox]

There is no reason an AttrValue should change variant once set, so we should refactor the API accordingly. This allows us to have better consistent panic messages in such cases.

[psdh]

Is this what you intend

match *self {`
    AttrValue::Atom(ref value) => Some(value),
    _ => panic!(),
}

[nox]

@psdh Yes.

[nox]

Actually no, without the Some(_). And with a message passed to panic!().

[tafia]

What is the reason to prefer panic over Result ?