move serializable to util directory

lib/fetching/cacheable.dart

 import 'dart:async';
 
-import '../serializable.dart';
+import '../util/serializable.dart';
 import '../untis/fetch.dart';
 
 abstract class Cacheable<T> {

lib/fetching/pageconfig.dart

-import '../serializable.dart';
+import '../util/serializable.dart';
 import '../util/date_extension.dart';
 import '../untis/element_type.dart';
 import '../untis/fetch.dart';

lib/fetching/timetable.dart

-import '../serializable.dart';
+import '../util/serializable.dart';
 import '../util/date_extension.dart';
 import '../untis/element_type.dart';
 import '../untis/fetch.dart';

lib/untis/element.dart

-import '../serializable.dart';
+import '../util/serializable.dart';
 
 mixin UntisMixin {
   Json get json;

lib/untis/fetch.dart

@@ -6,7 +6,7 @@ import 'package:flutter/foundation.dart';
 import '../fetching/pageconfig.dart';
 import '../fetching/timetable.dart';
 import 'class.dart';
-import '../serializable.dart';
+import '../util/serializable.dart';
 import 'element_type.dart';
 import 'lecture.dart';
 

lib/untis/lesson.dart

 import 'package:flutter/material.dart';
 
-import '../serializable.dart';
+import '../util/serializable.dart';
 import 'element.dart';
 import 'lecture.dart';
 import 'room.dart';

lib/user/profile.dart

-import '../serializable.dart';
+import '../util/serializable.dart';
 import '../util/listenable_set.dart';
 
 class Profile with Serializable {