Performance regression/Benchmark testing

[pierrejoye]

It may be good to have a minimal benchmark testing suite.

Maybe https://github.com/google/benchmark/blob/main/docs/user_guide.md would be good. We only need to add extern C around a function implementing the tests. It includes a comparison script as well, so we could store the standard run in a repo and fetch it to detect regressions.

Thoughts? Know any other tool we could use?