These are stubbed out and have type signatures but are currently unimplemented:

hash : HashAlgorithm -> a -> Bytes
hmac : HashAlgorithm -> Bytes -> a -> Bytes

I wanted to do some refactoring of how foreign data types are handled before implementing this, and also it needs to have a story for how continuations are hashed (which is probably similar to how continuations get serialized). I thought it was out of scope for #1712 so punted it to this issue.