pub fn chain_step_feed_kernel(block: [b8; 64]) -> [b8; 18]Expand description
The 18 input bytes that the feed-forward XORs back into the digest.
Input: the 64-byte chain block. Output: block[DIGEST_BYTES[i]] for
i in 0..18. Pure wiring. The lane saves these 18 bytes (not all 64)
and carries them down the pipeline next to the state, 20 stages deep.