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/playground/src/jsonp/jsonp_comp.ts

@@ -7,7 +7,7 @@ import {ObservableWrapper} from 'angular2/src/facade/async';
   template: `
     <h1>people</h1>
     <ul class="people">
-      <li *ngFor="#person of people">
+      <li *ngFor="let person of people">
         hello, {{person['name']}}
       </li>
     </ul>