If #4085 merges we need to do a few follow ups

• Revisit whether we need the Hash bound on HmacEngine::Hash or whether something weaker (AsRefSelf::Bytes, or, somehow, nothing)
• Revisit the Bytes type and see if we can't replace it with an associated constant (I remember we couldn't before, but things have changed now)
• Revisit which functions we should add to hmac and hkdf which are skipped in this PR since they're missing in the existing API

Original post: #4085 (comment)