Move internal modules to “internal” folder.

.internal/setToArray.js

@@ -0,0 +1,18 @@
+/**
+ * Converts `set` to an array of its values.
+ *
+ * @private
+ * @param {Object} set The set to convert.
+ * @returns {Array} Returns the values.
+ */
+function setToArray(set) {
+  let index = -1;
+  const result = Array(set.size);
+
+  set.forEach(value => {
+    result[++index] = value;
+  });
+  return result;
+}
+
+export default setToArray;