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/benchmark/transform/bind_generator/simple.dart

@@ -25,7 +25,7 @@ const aContents = '''
 library bar.ng_deps.dart;
 
 import 'bar.dart';
-import 'package:angular2/src/core/annotations/annotations.dart';
+import 'package:angular2/src/core/metadata.dart';
 
 bool _visited = false;
 void initReflector(reflector) {