Facilitate primary key rotation when revoking keys

For key revocations, there should be a subpacket "SuceededBy" to facilitate primary key rotation. Obviously, it should only be honored when used with the soft revocation reason "key superseded".

I think the subpacket should contain a fingerprint. Additionally, an implementation MAY chose to include a trust signature delegating to the new key in an embedded signature subpacket, or MAY chose to include the new certificate (which MAY include the tsig) in an related certificate subpacket.