We have both HASH_LEN and DIGEST_LEN

These are the same constant. Oh my god they aren't the same

We should use one of them consistently.

Also this name is a bit of a hostage to fortune since we'll want to change hash function and we certainly won't be able to do that by changing the definition HASH_LEN. So maybe neither of these is a good name. Indeed not!

spec/tor-spec/preliminaries.md:HASH_LEN=20.
spec/tor-spec/create-created-cells.md:DIGEST_LEN = MAC_LEN = MAC_KEY_LEN = ENC_KEY_LEN = PUB_KEY_LEN = 32

!312 (comment 3127299)