Aliased Array: Assign array entries that point to other entries

 * Extends ArrayObject to implement a aliased key array. Allows for any number of
 * unique keys (alias keys) pointing to an array element.
 *
 * CAUTION:  This will not handle multi-dimension arrays like you can't
 * do this:
 *
 *      $a = new AliasedArray();
 *      $a[1][2] = 25;
 *
 * This will not work and will not generate an error.
 *
 * To add an element you first have to add an indexed data element
 *
 *      $a = new AliasedArray();
 *      $a->set(key, data);
 *      $a[key] = data;
 *
 * You can also add an existing array:
 *
 *      $a = new AliasedArray($existingArray);
 *
 * After an element has been added you can add an alias for the key. The
 * alias is not allowed to match a key.
 *
 *      $a->alias(key, alias);
 *
 * The following would throw an exception:
 *
 *      $a->set('a', 10);
 *      $a->set('b', 20);
 *      $a->alias('a', 'b');
 *
 * because the alias 'a' is already used as a key.
 * If the key does not exist, an exception will be thrown.
 * So for example these will work:
 *
 *      $a->set(1, 'thedata');
 *      $a->alias(1, 'theabc');
 * or
 *      $a['key1'] = 'newdata';
 *      $a->alias('key1', 'alias1');
 *
 * To get the data
 *
 *      $a->get(1);         // returns 'thedata'
 *      $a[1];              // ditto
 *      $a['key1'];         // returns 'newdata'
 *      $a['alias1'];       // ditto
 *
 * if the element for the key or the index does not exist then a
 * NULL will be returned.  You can unset key and alias(es).  If you unset
 * by key then the key/value will be unset and any aliases pointing to the
 * value will be unset.  If you unset an alias only that alias will be unset.
 *
 *      unset($a['key1']);
 *      unset($a['alias1']);
 *
 * Iterators iterate across the key array and ignore the alias array.