* Add state destroy to SHA2 API

* Include optimized SPHINCS+ implementations

I've generated new implementations from the sphincsplus repository.

* Don't destroy sha256ctx after finalize

* Attempt to shut up MSVC

* Make sure to drop errors in rmtree

* Names passed around (mostly constant) value more clearly
* Put ``const`` where applicable (almost everywhere)
* Renamed ``primitive.h``
* Fixed some nits

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.