Get rid of 'class object' usages in code and builtins

Replace some comments and library usages as well

core/builtins/native/kotlin/String.kt

@@ -21,7 +21,7 @@ package kotlin
  * implemented as instances of this class.
  */
 public class String : Comparable<String>, CharSequence {
-    class object Companion {}
+    companion object {}
     
     /**
      * Returns a string obtained by concatenating this string with the string representation of the given [other] object.