improve NTT performance of avx2 ml-dsa

[franziskuskiefer]

• [ML-DSA] AVX2 performance improvements in NTT #584
• Things to investigate (increasing order of effort)
  • try different instructions for shuffling (e.g. vpshufd vs vmovshdup)
  • additionally unroll layers 2 through 0
  • use a different shuffling strategy altogether (i.e. instead of shuffle-in -> butterfly -> shuffle-out, per layer in layers 2 - 0, shuffle once in layers 5-3 and unshuffle when writing out the final result (?))
• Apply effective optimizations to inverse NTT
• Puzzle, potential waste of time: In our multiplication, subtractions seem to be disproportionately expensive although they shouldn't be, going from instruction count. Why is that?