pub fn fe25519_mul(a: &[u64; 4], b: &[u64; 4]) -> Option<[u64; 4]>Expand description
Multiplication modulo p = 2^255 - 19 on 4 x 64-bit limbs, for any inputs below 2^256: the
result is below 2^256 and congruent to a * b (not fully reduced). Constant time; None
without MULX/ADX. The high half of the product is folded in times 38 (2^256 = 38 mod p).