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/todo/todo.html

@@ -18,7 +18,7 @@
 
     <ul id="todo-list">
 
-      <li *ngFor="#todo of todoStore.list">
+      <li *ngFor="let todo of todoStore.list">
 
         <div class="view"
             [class.hidden]="todoEdit == todo">