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/zippy_component/index.ts

@@ -9,7 +9,7 @@ import {Zippy} from './zippy';
       This is some content.
     </zippy>
     <ul>
-      <li *ngFor="var log of logs">{{log}}</li>
+      <li *ngFor="let  log of logs">{{log}}</li>
     </ul>
   `,
   directives: [Zippy]