make a data type that can be either a key ID or a fingerprint
With OpenPGP v4 signatures (including certifications) there are several situations where there might be an issuer subpacket (containing a 64-bit key ID) and/or an issuer fingerprint subpacket (containing a full fingerprint).
It would be good to have a datatype capable of representing such a handle in whatever form is present -- either a key ID, or a fingerprint.
(this is split out of discusson over on #277 (moved))