Make Cmdliner.Arg.conv abstract and remove deprecated interfaces for 2.0

[dbuenzli]

This issue keeps track of breaking changes for 2.0 these are:

1. The commit that makes Cmdliner.Arg.conv abstract. Announced it would become in 2017, sadly it was not possible to visibly deprecate it.
2. Commits that remove interfaces that have been deprecated before 2.0.

Instructions

Projects can handle all breaking changes performed by 2.0 and still compile with cmdliner.1.3.0.

If you have been notified that your project will not compile with 2.0 and want to make sure it does please follow these instructions:

1. Compile your project with opam pin add cmdliner.1.3.0 and fix the deprecation warnings as suggested. These may not show up if you silenced them in which case either unsilence them or go to 2. they will show up as "Unbound value" errors.

2. Compile your projects with opam pin add cmdliner --dev and fix remaining compilation errors. These should mainly be due to the Arg.conv becoming an abstract type and can be tackled:

   • Tweaking your polymorphic variant result type into regular result variants and use either Arg.conv' or Arg.conv on your pairs.
   • Use the accessors Arg.conv_parser and Arg.conv_printer if you were deconstructing Arg.conv values as pairs.

3. Update your opam file with cmdliner {>= "1.3.0"}

Tracking a few project updates

Tracking a few project updates:

• Remove compatcmdliner and depend on cmdliner >= 1.3.0. ocaml/odoc#1328
• Handle cmdliner deprecation and support for cmdliner 2.0 OCamlPro/ocp-indent#328
• Handle cmdliner deprecation and support for cmdliner 2.0 OCamlPro/ocp-index#175

See also the references below.