Create new src/arithmetic.c for field-independent arithmetic (eg batch invert, is_square).

Replace p448_ with field_ where possible.

Create constant EDWARDS_D = -39081.

Create inline function field_mulw_scc for multiplying by compile-time signed curve constants.

src/include/barrett_field.h:

- Requires review: corrected failure to cast to (mask_t) prior to negation. (Or, if this is wrong; should cast to needed bitwidth explicitly.)
- Changed type of nwords_out to uint32_t to agree with header.

src/include/intrinsics.h:

- Fixed up various preprocessor statements to check for definition rather than value of built-ins.
- Added macro to use Clang’s __builtin_readcyclecounter on platforms on which it’s available. (Which is most platforms these days.)

src/include/magic.h: Preprocessor “if” versus “if defined”.

src/include/word.h: Fixed ifdefs; enabled support for memset_s on Darwin. Added explicit cast to mask_t.

Added void to function definitions and declarations in the following files (not including void is okay in modern C++, but not modern C, IIRC):

include/goldilocks.h, src/crandom.c, src/goldilocks.c, src/include/api.h, src/include/intrinsics.h, test/bench.c, test/test.c, test/test.h, test/test_arithmetic.c, test/test_goldilocks.c, test/test_pointops.c, test/test_scalarmul.c, test/test_sha512.c

Trying to work around an apparent GCC bug on SSE2, thanks Samuel
Neves.

Added an experimental NEON arch.  It's fast.  It's not yet GCC clean.
It needs some more work on general cleanliness too.

(you knew this would happen).

Added ARM NEON support.

Added support for precomputation on public keys, which speeds up
later signatures and ECDH calls.  See history.txt or the doc for
details.

Reworked internals so that private keys can be derived from any
32-byte secret random value.  This also means that secret keys
can be "compressed" for cold storage.

Added more tests.  Running the tests now requires GMP, though
Goldilocks itself does not.

Linking now uses visibility instead of exported.sym.