Assemblage as a tool

The `configure.ml` and `describe.ml` tools are gone. `assemblage` takes over with corresponding sub-commands I hope everybody listening looks forward to write the NSFW `ass^TAB` thousands of time in their life.

This series of commit also starts with the work of inverting the inversion of control. The mutable project list is gone. The tools, now called commands and in the new `As_cmd` compilation unit are not exposed anymore, but they should make a come back, possibly as extensible cmdliner terms.

For the fun (haha) I  also rewrote the bootstrap procedure to get rid of ocamlbuild which makes it four time faster on a cold build. 

Commits have a few more details.