Move internal modules to “internal” folder.

.internal/overArg.js

@@ -0,0 +1,13 @@
+/**
+ * Creates a unary function that invokes `func` with its argument transformed.
+ *
+ * @private
+ * @param {Function} func The function to wrap.
+ * @param {Function} transform The argument transform.
+ * @returns {Function} Returns the new function.
+ */
+function overArg(func, transform) {
+  return arg => func(transform(arg));
+}
+
+export default overArg;