use mm0 instead of 'a': ~0.002s faster
use mm1 instead of 'b': no apparent change
use mmx for c and d: no apparent change
use mmx for e and f: still no apparent change
use mmx for g and h: +0.003 slowdown
hard-coded final digest +=. (back to ~0.097s after all this, so no net change).
mm_ -> xmm_
draft of sha256 in j