Move internal modules to “internal” folder.

.internal/baseSetToString.js

@@ -0,0 +1,21 @@
+import constant from './constant.js';
+import identity from './identity.js';
+
+/**
+ * The base implementation of `setToString` without support for hot loop shorting.
+ *
+ * @private
+ * @param {Function} func The function to modify.
+ * @param {Function} string The `toString` result.
+ * @returns {Function} Returns `func`.
+ */
+function baseSetToString(func, string){
+  return Object.defineProperty(func, 'toString', {
+    'configurable': true,
+    'enumerable': false,
+    'value': constant(string),
+    'writable': true
+  });
+}
+
+export default baseSetToString;