Change "Iterable" interface to "Traversable"

The interface originally implemented in the UString class documented in                                                                                                                                                                                                                                                     
the `README.md` file was an invalid "Iterable" interface.                                                                                                                                                                                                                                                                   
PHP has no "Iterable" interface, however a "Traversable" interface does                                                                                                                                                                                                                                                     
exist (and seems to have been the intended choice.