Merge branch 'mdw/ec-ptcmpr'

* mdw/ec-ptcmpr:
  math/: Support EC2OSP and OS2ECP operations, with point compression.
  math/f-{prime,niceprime}.c: Fix reduction for `add', `sub' and `neg'.
  math/mp-modsqrt.c: Zero has a square root.  Return it correctly.
  math/: Improve some commentary in the binary-field arithmetic.