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.scala
class Context
opaque type R[A] = Context ?=> A
object R:
  def apply[A](a: A): R[A] = a

// A.scala
val num1: R[Int] = R(42) // ok
val num2: R[Int] = 42 // error because R[A] is opaque

given Context = new Context()
val num3: Int = num1 + 1 // ok, evaluation should work for opaque types