teach 'create from usage' to add 'operator' modifier when necessary

idea/testData/quickfix/createFromUsage/createFunction/iterator/createIteratorFromUsage1.kt.after

@@ -1,6 +1,6 @@
 // "Create member function 'iterator'" "true"
 class Foo<T> {
-    fun iterator(): Iterator<T> {
+    operator fun iterator(): Iterator<T> {
         throw UnsupportedOperationException("not implemented") //To change body of created functions use File | Settings | File Templates.
     }
 }

idea/testData/quickfix/createFromUsage/createFunction/iterator/createIteratorFromUsage2.kt.after

@@ -1,6 +1,6 @@
 // "Create member function 'iterator'" "true"
 class Foo<T> {
-    fun iterator(): Iterator<String> {
+    operator fun iterator(): Iterator<String> {
         throw UnsupportedOperationException("not implemented") //To change body of created functions use File | Settings | File Templates.
     }
 }