* 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

clang9.0.0 (https://releases.llvm.org/9.0.0/tools/clang/docs/ReleaseNotes.html)
adds a new satic analyzer: security.insecureAPI.DeprecatedOrUnsafeBufferHandling
which throws warnings if you use "unsafe" buffer handling functions which
includes memset and memcpy.
We have memset and mempy all over the place, so I think it's best to ignore this warning.
All the occurences that I looked at seemed perfectly "safe" to me.

* Ignore pointless clang-tidy warning.

It wants you to write type suffixes for integers using uppercase. Let's
just disable this warning.

https://clang.llvm.org/extra/clang-tidy/checks/readability-uppercase-literal-suffix.html

* Clean up bugprone-narrowing-conversion

Also fixes some tidy issues

Not sure why my local clang-tidy doesn't complain