Function montgomery_reduction_inner 

const fn montgomery_reduction_inner(
    upper: &mut [Limb],
    lower: &mut [Limb],
    modulus: &[Limb],
    mod_neg_inv: Limb,
) -> Limb

Algorithm 14.32 in Handbook of Applied Cryptography https://cacr.uwaterloo.ca/hac/about/chap14.pdf