Move internal modules to “internal” folder.

.internal/mapToArray.js

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