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/benchmarks/src/largetable/largetable_benchmark.ts

@@ -214,22 +214,22 @@ class CellData {
   template: `
       <table [ngSwitch]="benchmarkType">
         <tbody template="ngSwitchWhen 'interpolation'">
-          <tr template="ngFor #row of data">
-            <td template="ngFor #column of row">
+          <tr template="ngFor let row of data">
+            <td template="ngFor let column of row">
               {{column.i}}:{{column.j}}|
             </td>
           </tr>
         </tbody>
         <tbody template="ngSwitchWhen 'interpolationAttr'">
-          <tr template="ngFor #row of data">
-            <td template="ngFor #column of row" attr.i="{{column.i}}" attr.j="{{column.j}}">
+          <tr template="ngFor let row of data">
+            <td template="ngFor let column of row" attr.i="{{column.i}}" attr.j="{{column.j}}">
               i,j attrs
             </td>
           </tr>
         </tbody>
         <tbody template="ngSwitchWhen 'interpolationFn'">
-          <tr template="ngFor #row of data">
-            <td template="ngFor #column of row">
+          <tr template="ngFor let row of data">
+            <td template="ngFor let column of row">
               {{column.iFn()}}:{{column.jFn()}}|
             </td>
           </tr>