This is largely a save-your-work checkin.

Created p521/arch_ref64 code to make sure E-521 basically works.
Fixed some of the testing code around E-521.  It doesn't quite pass
everything yet.

Created p521/arch_x86_64 code with optimized multiply.  In this
checkin, the multiply is fast and works, but all the other code in
that directory is the completely unoptimized ref64 build which
reduces after every add and sub.  So the whole thing isn't fast yet.

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.