feat(router): introduce ParamMap to access parameters

The Router use the type `Params` for all of:
- position parameters,
- matrix parameters,
- query parameters.

`Params` is defined as follow `type Params = {[key: string]: any}`

Because parameters can either have single or multiple values, the type should
actually be `type Params = {[key: string]: string | string[]}`.

The client code often assumes that parameters have single values, as in the
following exemple:

```
class MyComponent {
sessionId: Observable<string>;

constructor(private route: ActivatedRoute) {}

ngOnInit() {
    this.sessionId = this.route
      .queryParams
      .map(params => params['session_id'] || 'None');
}
}

```

The problem here is that `params['session_id']` could be `string` or `string[]`
but the error is not caught at build time because of the `any` type.

Fixing the type as describe above would break the build because `sessionId`
would becomes an `Observable<string | string[]>`.

However the client code knows if it expects a single or multiple values. By
using the new `ParamMap` interface the user code can decide when it needs a
single value (calling `ParamMap.get(): string`) or multiple values (calling
`ParamMap.getAll(): string[]`).

The above exemple should be rewritten as:

```
class MyComponent {
sessionId: Observable<string>;

constructor(private route: ActivatedRoute) {}

ngOnInit() {
    this.sessionId = this.route
      .queryParamMap
      .map(paramMap => paramMap.get('session_id') || 'None');
}
}

```

Added APIs:
- `interface ParamMap`,
- `ActivatedRoute.paramMap: ParamMap`,
- `ActivatedRoute.queryParamMap: ParamMap`,
- `ActivatedRouteSnapshot.paramMap: ParamMap`,
- `ActivatedRouteSnapshot.queryParamMap: ParamMap`,
- `UrlSegment.parameterMap: ParamMap`

packages/router/src/index.ts

@@ -20,7 +20,7 @@ export {ExtraOptions, ROUTER_CONFIGURATION, ROUTER_INITIALIZER, RouterModule, pr
 export {RouterOutletMap} from './router_outlet_map';
 export {NoPreloading, PreloadAllModules, PreloadingStrategy, RouterPreloader} from './router_preloader';
 export {ActivatedRoute, ActivatedRouteSnapshot, RouterState, RouterStateSnapshot} from './router_state';
-export {PRIMARY_OUTLET, Params} from './shared';
+export {PRIMARY_OUTLET, ParamMap, Params} from './shared';
 export {UrlHandlingStrategy} from './url_handling_strategy';
 export {DefaultUrlSerializer, UrlSegment, UrlSegmentGroup, UrlSerializer, UrlTree} from './url_tree';
 export {VERSION} from './version';