Script refactoring, frontend: Treat scripts as classes (as opposed to function bodies)

ScriptDescriptor implements ClassDescriptor
Scripts are accessible in IDE via stub index
Replace special treatment of scripts with generic mechanism in several cases
Scripts to longer generate 'rv' property for storing result value (changes in repl required)

compiler/tests/org/jetbrains/kotlin/test/util/RecursiveDescriptorProcessor.java

@@ -122,7 +122,7 @@ public class RecursiveDescriptorProcessor {
 
         @Override
         public Boolean visitScriptDescriptor(ScriptDescriptor scriptDescriptor, D data) {
-            return visitClassDescriptor(scriptDescriptor.getClassDescriptor(), data);
+            return visitClassDescriptor(scriptDescriptor, data);
         }
 
         @Override