Include strong hash over target primary key

As per a suggestion by @dkg:

We could include an extra field in the subpacket that contains a hash calculated over the target primary key as if it were a fingerprint, but using the same hash algorithm as the containing signature packet. This compensates for any weakness resulting from an outdated fingerprint hash algorithm in the target key. This should then be cryptographically equivalent to a signature made directly over the target key material and should therefore be sufficiently strong for key equivalence (when used in a bidirectional manner as per #4 (closed)).