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

⚠️ This function is not part of the stable API.

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