MWCryptRand already has some useful utility functions wrapping PHP's
hash() and hash_hmac(). Let's make them public so we can use them from
other code.
But since "MWCryptRand" isn't really a good place for hashing functions,
let's move them to "MWCryptHash" instead.
Change-Id: I7542c719ac72beba7b0f6aa170bdb4c69fa6beab
