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/routing/drafts.html

@@ -2,7 +2,7 @@
   <h2 class="page-title">Drafts</h2>
 
   <ol class="inbox-list">
-    <li *ngFor="#item of items" class="inbox-item-record">
+    <li *ngFor="let item of items" class="inbox-item-record">
       <a id="item-{{ item.id }}"
          [routerLink]="['/DetailPage', {'id':item.id}]">
         {{ item.subject }}</a>

modules/playground/src/routing/inbox.html

@@ -2,7 +2,7 @@
   <h2 class="page-title">Inbox</h2>
 
   <ol class="inbox-list">
-    <li *ngFor="#item of items" class="inbox-item-record">
+    <li *ngFor="let item of items" class="inbox-item-record">
       <a id="item-{{ item.id }}"
          [routerLink]="['/DetailPage', {'id':item.id}]">{{ item.subject }}</a>
     </li>