Delta for AVX2

[fulmicoton]

As suggest by `` on reddit, compute_delta as well as the integration on AVX2 would be more efficient if it was relying on the `_mm256_alignr_epi8` instruction.

Copy pasting the incomplete snippet from reddit.

fn compute_delta(curr: DataType, prev: DataType) -> DataType {
    unsafe {
        let shifted = _mm256_alignr_epi8(curr, _mm256_permute2x128_i256(prev, curr, 0x21), 12);
        _mm256_sub_epi32(curr, shifted)
    }
}