This is a backport of the chainhash package made in Decred along with a
few additional things cleaned up, finished test coverage, and rewording
of some documentation to make it more generic.
In particular, the new package provides the definition of the hash type
and associated hashing functions which will allow the rest of the code to be
agnostic to the specific hash algorithm.
This only implements the package and does not change any of the code
base over to use it.
