[WIP] Add intel simd

[Raimo33]

This adds sse2 and avx2 support to the library in general, as discussed in #1700, wherever it yields an improvement as per the benchmarks.

arm has different SIMD instruction set; it would be nice to have a separate PR implementing that as well. Maybe after this is merged...

Tasks:

• Add 2 CI/CD flows with permutations of -msse2, -mno-sse2 when building for amd32
• Add 4 CI/CD flows with permutations of -mavx2, -msse2, -mno-avx2, -mno-sse2 when building for amd64
• Precompute vectors at startup (the ones marked with TODO: precompute )

Test & Benchmark

To reproduce the following results I temporarily added 3 scripts for building, testing, benchmarking as well as a jupyter notebook to visualize results.
You can verify yourself by running: ./simd-build.sh && ./simd-test.sh && ./simd-bench.sh and executing the notebook as is.

Results

[Raimo33]

To precompute simd constants at the start, the best solution I found was doing something like this:

#ifdef __SSE2__
  static __m128i _128_vec_ones;
#endif

CONSTRUCTOR void simd_init(void)
{
#ifdef __SSE2__
  _128_vec_ones   = _mm_set1_epi8('1');
#endif
}

where CONSTRUCTOR is __attribute__((constructor))

[Raimo33]

I'm constantly getting these warnings. Apparently they're harmless since I always use loadu and storeu, but for some reason the compiler doesn't like them.

warning: cast increases required alignment of target type [-Wcast-align]
  653 |         _mm256_storeu_si256((__m256i *)r->v, out);

The only fixes I found are:

1. aligning everything to 64bytes (impossible, breaks even some of my avx logic)
2. suppress the warning globally
3. suppress the warning inline each time