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/examples/core/pipes/ts/slice_pipe/slice_pipe_example.ts

@@ -22,7 +22,7 @@ export class SlicePipeStringExample {
 @Component({
   selector: 'slice-list-example',
   template: `<div>
-    <li *ngFor="var i of collection | slice:1:3">{{i}}</li>
+    <li *ngFor="let  i of collection | slice:1:3">{{i}}</li>
   </div>`
 })
 export class SlicePipeListExample {