Function packed_mod_add 

pub fn packed_mod_add<const WIDTH: usize>(
    a: &[u32; WIDTH],
    b: &[u32; WIDTH],
    res: &mut [u32; WIDTH],
    _p: u32,
    scalar_add: fn(u32, u32) -> u32,
)

Add two arrays of integers modulo P using packing.

This is a fallback which should only be compiled in situations where packings are unavailable.

Assumes that P is less than 2^31 and a + b <= 2P for all array pairs a, b. If the inputs are not in this range, the result may be incorrect. The result will be in the range [0, P] and equal to (a + b) mod P. It will be equal to P if and only if a + b = 2P so provided a + b < 2P the result is guaranteed to be less than P.

Scalar add is assumed to be a function which implements a + b % P with the same specifications as above.