Simplify trees

[odersky]

Get rid of Pair and SelectorFromTypeTree nodes. Neither had an important role to play, so the result is shorter and simpler code than what we had before.

[DarkDimius]

👍 It would be nice to document the change that now selectors of Imports are Thickets, not Pairs.

[odersky]

@DarkDimius It's in the doc comment of the Import tree.

[odersky]

@DarkDimius Do you want to do the review?

[smarter]

LGTM, though it may be nicer to have an extractor for an import rename:

  object ImportRename {
    def unapply[T >: Untyped](thicket: Thicket[T]): Option[(Ident[T], Ident[T])] = thicket match {
      case Thicket((name @ Ident(_)) :: (rename @ Ident(_)) :: Nil) => Some(name, rename)
      case _ => None
    }
  }

[odersky]

@smarter I am a bit concerned Import selector analysis might be time-critical, that's why I preferred the non-extractor solution, because it can be optimized better.