Add Cmd.alias to create aliases of commands

[kit-ty-kate]

I'm currently trying to port opam to use the 1.1 API and this would be incredibly useful to have (otherwise we need to carry the old (info, term) tuple around like the old API and it's not really pretty)

[dbuenzli]

If that works sufficiently well it doesn't feel egregious.

Another slight annoyance is to keep a term when you have it both a subcommand and as the default command but I don't have a solution for that (you'd like 'a Cmd.t -> 'a Term.t but the datastructure is gone).

[kit-ty-kate]

(you'd like 'a Cmd.t -> 'a Term.t but the datastructure is gone).

yeah that's what i looked for first but then i saw the new Cmd.t data structure and realized it wasn't possible