Move the unit sorting logic from Assemble to As_project.
Introduce Pkg.kind and add Assemblage.pkg_c.
Remove `Pkg_pp tag.
Kill these remainaing cu(s) they hurt my eye.
Move to non functional and named ~deps.

Somehow more verbose, but also clearer.
Redefine and rewrite Assemble.ocamldep.

Allow to interfere with the units construction.
Merge pull request #47 from dbuenzli/dep-improvements

Dep improvements