This PR sacrifices passing some extra arguments to get rid of the global
state.
* Haraka needs state in all hash calls, this results in changes to the
hash functions specified in `hash.h`. The extra pointers passed would
not be necessary for SHA256 or SHAKE256.
* SHAKE256 did not have global state, but uniformity in the implementations
requires us to pass around the new state context anyway. Otherwise,
@joostrijneveld's SPHINCS+ generator doesn't really work anymore).
We introduce a new header file called `primitive.h` which defines the
required state type for the generic functions. I did not go into
replacing _all_ occurrences of state variables by the new `hash_state`
macro.
