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/docs/core/10_view.md

@@ -94,7 +94,7 @@ Let's start with a View such as:
 
 ```
 <ul>
-  <li template="ngFor: #person of people">{{person}}</li>
+  <li template="ngFor: let person of people">{{person}}</li>
 </ul>
 ```