Drop the length argument from all hash and mac digest functions
In the late 1990s, it seems like a useful feature to support truncated hash functions (e.g., hmac-sha1-96 being a recommended MAC in the ssh protocol). That's no longer the case, it will simplify both interfaces and code to have each digest function have a fixed algorithm-dependent output size, and leave an extra memcpy to any applications that wants only the first part of the digest.
Length argument would be kept only for constructions like shake that produce a digest of arbitrary size.