Distribution of revocations

There's an open question in hockeypuck[1] about how to distribute third-party certification revocations after 1pa3pc is implemented. Bob may not be able or willing to attest to Alice's revocation over his key. I've called this out as a FIXME in !7 (merged) but will not work on it there (there's too much going on already).

My main idea is that Alice should be able to attach certification revocations made by her to her own key, to ensure distribution. This could be done using Embedded Signature subpackets in an existing or new signature type.

[1] https://github.com/hockeypuck/hockeypuck/wiki/HIP-4:-Third-party-revocation-sig-distribution