`ntt/src/ntt`: Replace by parallel alternative to speed up tests

[recmo]

On 2025-11-05 in ac42e37 “Merge pull request #227 from worldfnd/sl/duplicate-witness”:

Replace by parallel alternative to speed up tests

            prop_assert_eq!(original,s);
        }
    }

    // TODO Replace by parallel alternative to speed up tests
    fn transpose<T: Copy>(matrix: &[T], rows: usize, columns: usize) -> Vec<T> {
        assert_eq!(matrix.len(), rows * columns);
        let mut v = Vec::with_capacity(matrix.len());
        for j in 0..columns {
            for i in 0..rows {

From ntt/src/ntt.rs:434