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/template_driven_forms/index.ts

@@ -122,7 +122,7 @@ class ShowError {
       <p>
         <label for="country">Country</label>
         <select id="country" ngControl="country" [(ngModel)]="model.country">
-          <option *ngFor="#c of countries" [value]="c">{{c}}</option>
+          <option *ngFor="let c of countries" [value]="c">{{c}}</option>
         </select>
       </p>