Eliminate `CommonAST.universe` member

[aalexandrov]

The CommonAST trait has member universe and an alias u:

trait CommonAST {
  val universe: Universe
  lazy val u: universe.type = universe
  // ...
}

We should remove the long version and use only u. This removes certain problems with proper path dependent type resolution in IntelliJ.

@joroKr21 should guide this, if this is a lot I suggest to organize a synced refactoring and ask @emmalanguage/committers for help.

[aalexandrov]

I am seeing only 18 usages of universe member, I think I can give it a shot and append the changes to #345.