[K/N] Internalize Ref

As a part of efforts to stabilize Native stdlib.

kotlin-native/runtime/src/main/kotlin/kotlin/native/internal/Ref.kt

@@ -8,6 +8,7 @@ package kotlin.native.internal
 /**
  * This class is used to allocate closure-captured variables in the heap.
  */
-class Ref<T> {
+@PublishedApi
+internal class Ref<T> {
     var element: T = undefined()
 }