iterator.go: new interface ExtIterator derived from Iterator

iterator.go

@@ -13,6 +13,10 @@ type Iterator interface {
 	Next() (item any, err error) // must return io.EOF after the last item
 	Current() (item any, err error)
 	Index() int
+}
+
+type ExtIterator interface {
+	Iterator
 	HasOperation(name string) bool
 	CallOperation(name string, args []any) (value any, err error)
 }