Refactor for hax: avoid explicit return calls

[geonnave]

This is on top of #149.

[chrysn]

This is replacing an explicit panic from .get() with an implicit one.

If you want to cut down panic options, could replace the [1..buffer.len()] with [1..] as a start, but you can also take it further and start with

if let Ok((&first, tail)) = buffer.split_first() {
    /* use first */
    /* no need for buffer.len() > 1, but go right with: */
    if Ok((&label, slice)) = tail.split_first() {
        /* use CBOR decoder on label or err out */
        /* slice here contains exactly what the annotated line panicingly  */
    }
} else {
    Err(...)
}

[geonnave]

Yes, I thought of [1..] on the way back home 🙃, but not the second one. Updated.