+1 to adding a SHA1 SQL function. Even if it's deprecated, there's plenty of historical usage that I can see it being useful.
Either way, the rest of the refactor can be improved a bit to perform a single palloc() and remove the memcpy(). Attached is a diff for cryptohashfuncs.c that does that by writing the digest final directly to the result. It also removes the digest length arg and determines it in the switch block. There's only one correct digest length for each type so there's no reason to give callers the option to give the wrong one.
Regards,
-- Sehrope Sarkuni