Add structure for widget messaging layer

src/utils/iterables.ts

@@ -0,0 +1,21 @@
+/*
+ * Copyright 2020 The Matrix.org Foundation C.I.C.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *         http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { arrayUnion } from "./arrays";
+
+export function iterableUnion<C extends Iterable<T>, T>(a: C, b: C): Set<T> {
+    return new Set(arrayUnion(Array.from(a), Array.from(b)));
+}