Important work items for Ed448-Goldilocks / decaf / Ristretto.

* Test signed 32-bit NEON implementation to avoid bias/reduce after subtract

* Documentation: write high-level API docs, and internal docs to help
  other implementors.
    * Pretty good progress on Doxygenating the code.

* Documentation: help work on ristretto.group website.

* Cleanup: unify intrinsics code
    * Generate asm intrinsics with a script?

* Testing:
    * More testing.  Testing, testing and testing.
    * Test corner cases better.
    * Try to formally verify some things.

* Safety:
    * If RNG fails, return error and zeroize

* Portability: test and make clean with other compilers
    * Using a fair amount of __attribute__ code.
    * Should work for GCC now.
    * Autoconf

* Portability: try to make the vector code as portable as possible
    * Currently using clang ext_vector_length.
    * I can't get a simple for-loop to autovectorize :-/
    * Autogenerate somehow?
