Introduce KClass.members, make properties/extensionProperties extensions

To avoid significant growth of KClass and KPackage interfaces

core/builtins/src/kotlin/reflect/KClass.kt

@@ -39,12 +39,8 @@ public interface KClass<T> : KDeclarationContainer {
     public val qualifiedName: String?
 
     /**
-     * Returns non-extension properties declared in this class and all of its superclasses.
+     * All elements accessible in this class, including functions and properties
+     * declared in this class and all of its superclasses.
      */
-    public val properties: Collection<KProperty1<T, *>>
-
-    /**
-     * Returns extension properties declared in this class and all of its superclasses.
-     */
-    public val extensionProperties: Collection<KProperty2<T, *, *>>
+    public val members: Collection<KCallable<*>>
 }