Rudder

For now and the foreable future, cats has won as the standard category theory library for Scala.

We need to use it and upgrade all our libraries to use version based on cats. It seems to be possible.

Also, that would remove a BIG dependency (scalaz7 is 9Mo).

I believe we don't need cats (appart for test) in anything below rudder, but we need to add it in parent pom to ensure consistency of dependencies.