enable Continuous Integration

This MR offers to setup Continuous Integration for MRs on the central Inferno repository (in fact any clone of it), using the scripts from my project

https://gitlab.com/gasche/gitlab-ocaml-ci-example/

I had to find out how to enable gitlab CI on gitlab.inria.fr projects. The dedicated INRIA documentation is here. It fails to mention the most convenient way to do this, which is to use the qlf-ci-gitlab-runner runners (see their getting started); I heard about it by asking the Gitlab people on the INRIA Mattermost.