Say conf

True configuration system, simplification and clarification of the build model and concepts exposed, clear separation between user API, driver API and default driver. Still feels that a lot of work is needed to get to the right API and model, but this may be a good start. The project doesn't bootstrap yet, I didn't have the time to port all the old build logic to the new system, sorry about that but I'm really late now. Will follow up with a few issues on the BT from notes that I have locally.

P.S. Until I make the next release of cmdliner a pin on cmdliner's repo is is needed to compile the project.