AVX2: Add native implementation of `poly_caddq`

[jakemas]

[mkannwischer]

@jakemas. Now with #371 merged, this may look promising again. Can you hoist out the caddq from #325 into a new pull request so we can measure it?

[mkannwischer]

I extracted it and included it in #414 - need to wait for the benchmarks to see if it is worth it.