cmdliner 2.0.0 support

[dbuenzli]

It seems that the latest version of your package on the opam repository will not compile with the upcoming cmdliner 2.0.0 release.

This is either because you are using deprecated identifiers that are removed in 2.0.0 or because you still use argument converters as pairs which has been (sadly only silently) deprecated since 2017.

There are a few instruction here on how to make it compatible. If you run into problem or need more help use this issue, I'm listening.

[dbuenzli]

Sorry for the dupe. This is because there are two packages concerned: mirage.4.9.0 and functoria.4.4.2 whose opam dev-repo: metadata both point to this repo.

[hannesm]

Thanks for your report @dbuenzli. I tried to work on this in #1604, but the tests are now failing. Unfortunately I don't understand the code (functoria) well enough to have an idea what is the root issue. There's also quite some confusion (at least for me) what functoria attempts to achieve with using these cmdliner internals. If you have some spare time and want to take a look, that'd be wonderful. If you're busy with other things, we will have to wait (and put cmdliner constraints) until someone more knowledgeable (or patient) takes a look into this.