udpate to a recent cmdliner

[gasche]

Cmdliner 1.1 deprecated some parts of the Term module in favor of a more recent Cmd module, see

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

As a consequence, building dune-deps shows the following warning:

File "src/bin/main.ml", line 164, characters 4-13:
164 |     Term.info
          ^^^^^^^^^
Alert deprecated: Cmdliner.Term.info
Use Cmd.info instead.
File "src/bin/main.ml", line 169, characters 8-17:
169 |   match Term.eval (cmdline_term, info) with
              ^^^^^^^^^
Alert deprecated: Cmdliner.Term.eval
Use Cmd.v and one of Cmd.eval* instead.

This PR updates to the recommended 1.1.0 API. Unfortunately, as a result, the new code does not work with Cmdliner 1.0.x anymore. I updated the opam dependencies to reflect this.