Add --merlin=BOOL option to setup.
Don't bootstrap's assemblage's own .merlin.

Otherwise you can't use merlin before you were able to bootstrap,
which is very annoying.
Massive refactoring, api and internal changes.

The project compiles but the functionality is broken.
Add As_string.{cut,rcut,splice}.
Add module As_conf.
Add module As_cmd.
Make As_conf more useful. But it's not there yet.
Remove As_cond (Features) and Asd_setup_env. Start to propagate As_conf.
Simplify configuration to cmdliner term conversion.

Since we don't print the absent values (they can't be accurate, we
don't know the configuration) there's no need to create it in
dependency order.
Make logging more opam-like.
Improve configuration keys and their documentation system.

Also move the ad-hoc dumpast command line flag to a configuration
key.
Add and use Assemblage.Cmd.Vcs for accessing VCS information.
Remove Asd_shell, superseeded by Assemblage.{Cmd,Fmt}.
Reorganize source tree a bit.
Move a few things to the Assemblage.Private API.
Percolate the Conf.value in the system.
Forget about As_product. Use simple As_path.rel as Action IO.
Write basic OCaml product actions. Export them in the API.
Fly things around again.
Untangle assemblage parts.