Very experimental Ed480-Ridinghood support is now in.  It's not fully optimized,
but in general the current build is 8-15% slower than Goldilocks.  It only works on
arch_x86_64, though arch_ref64 support ought to be easy.  Support on other arches
will be trickier, which is of course why I chose Goldilocks over Ridinghood in the
first place.

Next up, E-521.  Hopefully.

The code is starting to get spread out over a lot of files.  Some are per field*arch,
some per field, some per curve, some global.  It's hard to do much about this, though,
with a rather ugly .c.inc system.

There's currently no way to make a Ridinghood eBAT.  In fact, I haven't tested eBAT
support in this commit.  I also haven't tested NEON, but at least ARCH_32 works on
Intel.

Undo the INTRINSIC changes from David Leon Gil.

Turn precomputed keys back on by default.  Change #ifdef to #if for checking sigs.