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/router/ts/on_deactivate/on_deactivate_example.ts

@@ -36,7 +36,7 @@ class MyCmp implements OnDeactivate {
     <router-outlet></router-outlet>
     <div id="log">
       <h2>Log:</h2>
-      <p *ngFor="#logItem of logService.logs">{{ logItem }}</p>
+      <p *ngFor="let logItem of logService.logs">{{ logItem }}</p>
     </div>
   `,
   directives: [ROUTER_DIRECTIVES]