main: Make the code compile

[ilankri]

To do so, we need to switch to Cmdliner 1.1.

See https://github.com/dbuenzli/cmdliner/blob/master/CHANGES.md#new-cmd-module-and-deprecation-of-the-term-evaluation-interface for migration guide.

[kit-ty-kate]

I squashed and renamed the commit to Switch to Cmdliner 1.1.0 as the code compiled fine before, it just returned deprecation warnings (which dune transforms into errors in dev mode). Thanks

[ilankri]

Indeed, thanks!