Function mul_redc 

pub fn mul_redc<const N: usize>(
    a: [u64; N],
    b: [u64; N],
    modulus: [u64; N],
    inv: u64,
) -> [u64; N]

Computes a * b * 2^(-BITS) mod modulus

Requires that inv is the inverse of -modulus[0] modulo 2^64. Requires that a and b are less than modulus.