most use of that API is to add a single sal_Unicode, it is
silly to manufacture a full OUString just to pass that via
AddToCode to then append it to a OUStringBuffer

adding AddToCode(sal_Unicode c) to simplify these case

also remove a silly iteration over a OUString's character to
re-add each character one by one via AddToCode()

Change-Id: Ia8a58551a1c24312baaa250b8d36fe21c46127e7