sha256.c: simplify byte reverse ordering and explicitly cast from ULL to unsigned char 1. Currently implemented reverse ordering looks surprisingly complex, but we can safely remove the subtraction part if we pass the current index to the state array and increase the loop 2x times more (4 * 2). It's the same as the previous implementation, except for the loop and index covers here, so the extra calculation (i * 8) in the loop, is no longer needed.
rilysh