Allow opaque type aliases of context functions

[scf37]

Motivation: Given opaque type R[A] = Context ?=> A, I want to allow type conversion R[A] -> A but deny A -> R[A]

// Context.scalaclassContextopaquetypeR[A] =Context?=>AobjectR:defapply[A](a: A):R[A] = a // A.scalavalnum1:R[Int] =R(42) // okvalnum2:R[Int] =42// error because R[A] is opaquegivenContext=newContext() valnum3:Int= num1 +1// ok, evaluation should work for opaque types