feat(compiler): introduce <ng-template>, deprecate <template> and template attribute

The rationale of this change is to improve the inter-operability with web
components that might make use of the `<template>` tag.

DEPRECATION

The template tags and template attribute are deprecated:

    <template ngFor [ngFor]=items let-item><li>...</li></template>
    <li template="ngFor: let item of items">...</li>

should be rewritten as:

    <ng-template ngFor [ngFor]=items let-item><li>...</li></ng-template>

Note that they still be supported in 4.x with a deprecartion warning in
development mode.

MIGRATION

- `template` tags (or elements with a `template` attribute) should be rewritten
as a `ng-template` tag,
- `ng-content` selectors should be updated to referto a `ng-template` where they
use to refer to a template: `<ng-content selector="template[attr]">` should be
rewritten as `<ng-content selector="ng-template[attr]">`
- if you consume a component relying on your templates being actual `template`
elements (that is they include a `<ng-content selector="template[attr]">`). You
should  still migrate to `ng-template` and make use of `ngProjectAs` to override
the way `ng-content` sees the template:
`<ng-template projectAs="template[attr]">`
- while `template` elements are deprecated in 4.x they continue to work.

modules/benchmarks/src/largetable/ng2_switch/table.ts

@@ -15,9 +15,9 @@ import {TableCell, emptyTable} from '../util';
   selector: 'largetable',
   template: `<table><tbody>
     <tr *ngFor="let row of data; trackBy: trackByIndex">
-    <template ngFor [ngForOf]="row" [ngForTrackBy]="trackByIndex" let-cell><ng-container [ngSwitch]="cell.row % 2">
+    <ng-template ngFor [ngForOf]="row" [ngForTrackBy]="trackByIndex" let-cell><ng-container [ngSwitch]="cell.row % 2">
         <td *ngSwitchCase="0" style="background-color: grey">{{cell.value}}</td><td *ngSwitchDefault>{{cell.value}}</td>
-    </ng-container></template>
+    </ng-container></ng-template>
     </tr>
   </tbody></table>`
 })