fix key hashing for v5 / v6 signatures

[TJ-91]

Justus pointed out that the hashing of keys is implemented incorrectly for RNP and some other implementations, see https://mailarchive.ietf.org/arch/msg/openpgp/PyP-XDv0VM5bYPX1Iq41-Oyytds/ This concerns computing v5 and v6 signatures over keys.

This PR fixes that. I separated signature and fingerprint computation logically, but the same code is used in the background. I also added a check that we don't accidently hash too large a key.