[build] [wip] Support for the dune build system v2

[hhugo]

Continue the effort started in #73.

In particular, it tries to address the following comment

To get a more decisive opinion and be excited about this, it would help to see a complete dune-based build system with the configure phase working on all the platforms currently supported by the Makefile

It's rebased on top of the recently merged new CI jobs.

List of changes:

• move sources in sub directories
  • src/ : source of zarith library
  • src/top/ : source of the toplevel printers
  • config/ : dune-configurator executable to compute flags used by the c compiler
• The meta and opam file is now generated by dune
• config/discover.ml replaces the configure script

[rgrinberg]

Is there anything else that needs to be done in this PR? It looks good to me.

[xavierleroy]

Is there anything else that needs to be done in this PR? It looks good to me.

First, the PR should not be marked Draft if you want me to look at it.

Second, I still don't understand what we have to gain by switching to a Dune build. This is a trivial project as far as OCaml sources are concerned, but it has a C side with non-trivial configuration and dependencies. That's precisely the kind of project where Dune does not shine. Why insisting on using Dune here?