Upstream changes from PQClean

[joostrijneveld]

The reference code as included in pqclean contains various small fixes that should still be upstreamed back to this repository. It's nothing major, but for the time being it may be nice to take that as a starting point instead.

[thomwiggers]

#7 lists at least the following changes that were necessary for PQClean, though the current status of this repository and PQClean have diverged quite a bit:

• Get rid of compiler-specific alignment, doing it generically instead
• Get rid of the global state (Thread safety #19)
• Add namespaces to everything (Namespace SPHINCS+ internals #37)
• no VLA arrays (Variable-length arrays are not compatible with MSVC #20)

[thomwiggers]

• Don't use hash function internals (Refactor FIPS/SHA2 APIs #40)