Add some convenience printers in OpamCudf

Now that the opam-name and opam-version are in the CUDF file, it's much easier to debug. Sweet!
Some tweaks to the heuristic to work on smaller universes.

This change makes opam-rt work again with the internal heuristics.
Merge pull request #1358 from samoht/master

Fix the internal solver to work decently again.