sigpubkey/sigprivkey: Provide a hash_if

Sometimes, the caller is going to want to hash things for themselves.
(Notably, site.c will want to hash the advertisements of public key
set updates.)

The hash to be used should be the one used by the signature scheme, as
that has the same security properties.

In rsa.c this moves the hash_if from common to ops, and it is now
convenient to abolish rsa_hash which is a pointless veneer over
hash_hash.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>