Add reduce iterables utility

ts/test-both/util/iterables_test.ts

@@ -11,6 +11,7 @@ import {
   groupBy,
   isIterable,
   map,
+  reduce,
   size,
   take,
 } from '../../util/iterables';
@@ -311,6 +312,23 @@ describe('iterable utilities', () => {
     });
   });
 
+  describe('reduce', () => {
+    it('returns the accumulator when passed an empty iterable', () => {
+      const fn = sinon.fake();
+
+      assert.strictEqual(reduce([], fn, 123), 123);
+
+      sinon.assert.notCalled(fn);
+    });
+
+    it('iterates over the iterable, ultimately returning a result', () => {
+      assert.strictEqual(
+        reduce(new Set([1, 2, 3, 4]), (result, n) => result + n, 89),
+        99
+      );
+    });
+  });
+
   describe('take', () => {
     it('returns the first n elements from an iterable', () => {
       const everyNumber = {