Create a benchmark script

[vgvassilev]

Our CI has already benchmark comparison action which compares the benchmarks against the submitted pull request:

https://github.com/vgvassilev/clad/blob/ec76b704ced2bb7471b25b51a7c24273374700d9/.github/workflows/ci.yml#L755-L781

We should turn that into a script benchmark_compare.py that can take two revisions and produce a comparison output in the same way. That'd allow us to run on local machines, not only in the CI.