Allow running benchmarks on pull requests

[gs0510]

The custom dockerfile allows installing system dependencies like zlib, the Makefile has the "make bench" target that is run once all the dependencies are installed and the opam file allows installing necessary dependencies for the benchmarks.

[gs0510]

We will also need to enable current-bench on this repo, once you take a look at this @dinosaure I can send you a request to approve running the benchmarks app.

[dinosaure]

Thanks, it seems fine for me. I will happy to accept your request and finally see benchmarks on this project!