refactor(core): introduce ViewRef and ProtoViewRef

BREAKING CHANGES:
- `NgElement` merged into `ElementRef`
- `Compiler.compile…` returns `ProtoViewRef`
- `ViewContainer` uses `ProtoViewRef`s and `ViewRef`s.
- `ViewRef`/`ProtoViewRef` in renderer were renamed to
  `RenderViewRef`/`RenderProtoViewRef`.

Related to #1477
Closes #1592

modules/benchmarks/src/tree/tree_benchmark.js

@@ -1,4 +1,4 @@
-import {bootstrap, Component, Viewport, View, ViewContainerRef, Compiler, NgElement, Decorator} from 'angular2/angular2';
+import {bootstrap, Component, Viewport, View, ViewContainerRef, Compiler, Decorator} from 'angular2/angular2';
 
 import {LifeCycle} from 'angular2/src/core/life_cycle/life_cycle';
 import {reflector} from 'angular2/src/reflection/reflection';