der: add test for derive Sequence with IMPLICIT SequenceRef

[dishmaker]

Depends on #1980

#[derive(Sequence)]
pub struct OidAndImplicitSequence<'a> {
    oid: ObjectIdentifier,

    #[asn1(context_specific = "0", tag_mode = "IMPLICIT")]
    data: SequenceRef<'a>,
}

[baloo]

This is one I don't understand.
How does that not break invariants?

SequenceRef::from_der should already work? I'm missing something.

[dishmaker]

from_der would parse tag and length first, then the contents.

[tarcieri]

Yeah, they do two different things: from_der would need the tag/length header, where this is just the "value" portion sans header

[baloo]

That makes more sense.
But then I would say the comment could probably use an update (as is, it would definitely have tripped me up).

Something like /// Create a new [SequenceRef] from the DER bytes of the inner value or something along those lines.