It's no longer needed since we require OCaml >= 4.08.

The cmdliner build system doesn't support parallel execution and
is prone to races (for instance, TOCTOU in `mkdir`).

Close #230

Close #249

Close #250

Close #214, close 245

Close #140

Close #231, close #244

Fix #231 point 1. Close #242

Close #238 #241

To help with testing dir/file/path completions (#238).

This allows for context sensitive completion to happen
regardless of position.

Incidentally we also get to complete --version and --help by
using the complete Cmd_info.t value.

This requires quite a bit of churn because it introduces
a type recursion.