Move internal modules to “internal” folder.

.internal/arrayReduce.js

@@ -0,0 +1,25 @@
+/**
+ * A specialized version of `reduce` for arrays.
+ *
+ * @private
+ * @param {Array} [array] The array to iterate over.
+ * @param {Function} iteratee The function invoked per iteration.
+ * @param {*} [accumulator] The initial value.
+ * @param {boolean} [initAccum] Specify using the first element of `array` as
+ *  the initial value.
+ * @returns {*} Returns the accumulated value.
+ */
+function arrayReduce(array, iteratee, accumulator, initAccum) {
+  let index = -1;
+  const length = array == null ? 0 : array.length;
+
+  if (initAccum && length) {
+    accumulator = array[++index];
+  }
+  while (++index < length) {
+    accumulator = iteratee(accumulator, array[index], index, array);
+  }
+  return accumulator;
+}
+
+export default arrayReduce;