moved SHA-512 source files to src/optional

moved SHA-512 source files to src/optional

There are 2 reasons behind this change:

- The primary way to install Monocypher is to copy the source files
  into one's own project.  But it wasn't clear whether `sha512.c` and
  `sha512.h` are meant to be copied as well.

- Monocypher is advertised as a single source file library (or a 2
  files library if you count the header), and a casual glance may
  disagree.

Now things should be much clearer.

---

I made another slight change to the vector generation process: I
removed the optimisation options, which in conjunction with `-std=c99`
seem to trigger a bug in GCC 5.4.0 (it can't find a type definition).
Clang works.

Those optimisation options slowed down the whole process anyway, so no
loss there.