Make reach refinement shallow

[Linyxus]

This is to address the following soundness issue:

trait File
val useFile: [R] -> (path: String) -> (op: File^ -> R) -> R = ???
def main(): Unit =
  val f: [R] -> (path: String) -> (op: File^ -> R) -> R = useFile
  val g: [R] -> (path: String) -> (op: File^{f*} -> R) -> R = f  // should be an error
  val leaked = g[File^{f*}]("test")(f => f)  // boom

[odersky]

I think the logic can be simplified a bit by this:

Suggested change

	case t1 @ CapturingType(p, cs) if cs.isUniversal =>
	case t1 @ CapturingType(p, cs) if cs.isUniversal && !isFlipped =>

[odersky]

Otherwise LGTM

[odersky]

Then you can leave things as they were here.