Function k_times_l_low 

pub fn k_times_l_low(k: b3) -> b128

Low 128 bits of k * l for k = 0..8 (the high 128 bits are k << 124).

k * l = k * 2^252 + k * c, and k * c < 8 * 2^125 = 2^128, so the low word is exactly k * c and the high word is k * 2^252 / 2^128. Entry 1 is c itself, which the finisher’s add-back step reuses.