feat(core): separate refs from vars.

Introduces `ref-` to give a name to an element or a directive (also works for `<template>` elements), and `let-` to introduce an input variable for a `<template>` element.

BREAKING CHANGE:
- `#...` now always means `ref-`.
- `<template #abc>` now defines a reference to the TemplateRef, instead of an input variable used inside of the template.
- `#...` inside of a *ngIf, … directives is deprecated.
  Use `let …` instead.
- `var-...` is deprecated. Replace with `let-...` for `<template>` elements and `ref-` for non `<template>` elements.

Closes #7158

Closes #8264

modules/angular2/test/core/debug/debug_node_spec.ts

@@ -132,9 +132,9 @@ class ConditionalParentComp {
 @Component({
   selector: 'using-for',
   viewProviders: [Logger],
-  template: `<span *ngFor="#thing of stuff" [innerHtml]="thing"></span>
+  template: `<span *ngFor="let thing of stuff" [innerHtml]="thing"></span>
             <ul message="list">
-              <li *ngFor="#item of stuff" [innerHtml]="item"></li>
+              <li *ngFor="let item of stuff" [innerHtml]="item"></li>
             </ul>`,
   directives: [NgFor, MessageDir],
 })