Clean up bn_sqr_words()

Clean up bn_sqr_words()

Currently there are two versions of bn_sqr_words(), which call the sqr or
sqr64 macro. Replace this with a single version that calls bn_umul_hilo()
and remove the various implementations of the sqr macro. The only slight
downside is that sqr64 does three multiplications instead of four, given
that the second and third terms are identical. However, this is a minimal
gain for the amount of duplication and entanglement it introduces.

ok tb@