* 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
There are schemes, like SABER (#192) that have more than one principal
submitter. Consistency warrants that we turn it into a list for all
schemes and don't do something with allowing either a str or a list:
that would just be very annoying to parse.
Closes#194
