Utility libraries to use with [pp]

[pplumbing] defines a set of utility libraries to use with pp. It is compatible with logs and inspired by design choices used by dune for user messages:

• [Pp_tty] extends pp to build colored documents in the user's terminal using ansi escape codes.

• [Err] is an abstraction to report located errors and warnings to the user.

• [Log] is an interface to logs using [Pp_tty] rather than [Format].

• [Log_cli] contains functions to work with [Err] on the side of end programs (such as a command line tool). It defines command line helpers to configure the [Err] library, while taking care of setting the logs and fmt style rendering.

• [Cmdlang_cmdliner_runner] is a library for running command line programs specified with cmdlang with cmdliner as a backend and making opinionated choices, assuming your dependencies are using [Err].

These libraries are meant to combine nicely into a small ecosystem of useful helpers to build CLIs in OCaml.