Move internal modules to “internal” folder.

.internal/objectToString.js

@@ -0,0 +1,19 @@
+/**
+ * Used to resolve the
+ * [`toStringTag`](http://ecma-international.org/ecma-262/7.0/#sec-object.prototype.tostring)
+ * of values.
+ */
+const nativeObjectToString = Object.prototype.toString;
+
+/**
+ * Converts `value` to a string using `Object.prototype.toString`.
+ *
+ * @private
+ * @param {*} value The value to convert.
+ * @returns {string} Returns the converted string.
+ */
+function objectToString(value) {
+  return nativeObjectToString.call(value);
+}
+
+export default objectToString;