Add continuous benchmarking

[ricardoboss]

There exists a GitHub action which can monitor performance diffs on a commit-to-commit basis: https://github.com/marketplace/actions/continuous-benchmark. They don't officially support phpbench yet, though.

Having a monitoring tool could have great effects on code performance in the long term.

Before continuous benchmarking is of any use, there need to be benchmarks that test actual performance-critical code paths (like routing or dependency injection).