refactor: rename annotations to metadata

BREAKING CHANGE (maybe)

Well as long as our customers use public API this should not be a
breaking change, but we have changed import structure as well as
internal names, so it could be breaking.

import:
  angular2/annotations => angular2/metadata

Classes:
  *Annotations => *Metadata
  renderer.DirectiveMetadata => renderer.RendererDirectiveMetadata
  renderer.ElementBinder => renderer.RendererElementBinder
  impl.Directive => impl.DirectiveMetadata
  impl.Component => impl.ComponentMetadata
  impl.View => impl.ViewMetadata

Closes #3660

modules/angular2/test/transform/integration/two_annotations_files/bar.dart

@@ -1,7 +1,6 @@
 library bar;
 
-import 'package:angular2/src/core/annotations_impl/annotations.dart';
-import 'package:angular2/src/core/annotations_impl/view.dart';
+import 'package:angular2/src/core/metadata.dart';
 
 @Component(selector: '[soup]')
 @View(template: 'Salad: {{myNum}} is awesome')

modules/angular2/test/transform/integration/two_annotations_files/expected/bar.ng_deps.dart

@@ -6,8 +6,7 @@ import 'package:angular2/src/change_detection/pregen_proto_change_detector.dart'
 import 'bar.dart';
 export 'bar.dart';
 import 'package:angular2/src/reflection/reflection.dart' as _ngRef;
-import 'package:angular2/src/core/annotations_impl/annotations.dart';
-import 'package:angular2/src/core/annotations_impl/view.dart';
+import 'package:angular2/src/core/metadata.dart';
 
 var _visited = false;
 void initReflector() {