feat(zipAll): add higher-order lettable version of zipAll

src/operator/zipAll.ts

@@ -1,5 +1,5 @@
-import { ZipOperator } from '../operators/zip';
 import { Observable } from '../Observable';
+import { zipAll as higherOrder } from '../operators/zipAll';
 
 /**
  * @param project
@@ -8,5 +8,5 @@ import { Observable } from '../Observable';
  * @owner Observable
  */
 export function zipAll<T, R>(this: Observable<T>, project?: (...values: Array<any>) => R): Observable<R> {
-  return this.lift(new ZipOperator(project));
+  return higherOrder(project)(this);
 }

src/operators/index.ts

@@ -80,3 +80,4 @@ export { windowToggle } from './windowToggle';
 export { windowWhen } from './windowWhen';
 export { withLatestFrom } from './withLatestFrom';
 export { zip } from './zip';
+export { zipAll } from './zipAll';

src/operators/zipAll.ts

@@ -0,0 +1,7 @@
+import { ZipOperator } from './zip';
+import { Observable } from '../Observable';
+import { OperatorFunction } from '../interfaces';
+
+export function zipAll<T, R>(project?: (...values: Array<any>) => R): OperatorFunction<T, R> {
+  return (source: Observable<T>) => source.lift(new ZipOperator(project));
+}