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/cheatsheet/built-in-directives.md

@@ -14,7 +14,7 @@ Removes or recreates a portion of the DOM tree based on the showSection expressi
 
 @cheatsheetItem
 syntax:
-`<li *ngFor="#item of list">`|`*ngFor`
+`<li *ngFor="let item of list">`|`*ngFor`
 description:
 Turns the li element and its contents into a template, and uses that to instantiate a view for each item in list.