Add a type printer for Scala compiler types.

This code follows closely what toString does in the Scala compiler, except
that it won’t print fully-qualified names. Most of the times, FQN are a nuisance
inside the IDE: They add clutter and little information, and when used from
quick-fix (add explicit type or implement abstract member), they need to be
manually removed.
Switch the quick fix to use the new printer (away with fully-qualified names!)
Switch the abstract member proposal to the new printer.