Guidance on selecting self-signatures

This change requires selecting the most recent valid self-signature on User IDs, keys and subkeys (certification signatures (types 0x10-0x13), the direct-key signature (type 0x1F), and the subkey binding signature (type 0x18) respectively - notably it doesn't apply to revocation signatures, which have different rules).

This allows keys to have an agreed-upon revocation date, among other properties.

(This MR cherry-picks a part of !134 (closed) that wasn't merged in !153 (merged).)

Closes: #103