Function mac_by_limb 

const fn mac_by_limb<const LIMBS: usize>(
    a: &Uint<LIMBS>,
    b: &Uint<LIMBS>,
    c: Limb,
    carry: Limb,
) -> (Uint<LIMBS>, Limb)

Computes a + (b * c) + carry, returning the result along with the new carry.