Description:
------------
A new interface called 'AppendableArrayAccess' that extends ArrayAccess (and be implemented by ArrayIterator and ArrayObject). This new interface is simply a push up of the append() method from ArrayIterator and ArrayObject.

Reproduce code:
---------------
<?php
class Example implements AppendableArrayAccess {
    public $array;

    public function __construct () {
        $this->array = array();
    }

    public function offsetGet($offset) {
        return $this->array[$offset];
    }

    public function offsetSet($offset, $value) {
        $this->array[$offset] = $value;
    }

    public function offsetExists($offset) {
        return isset($this->array[$offset]);
    }

    public function offsetUnset($offset) {
        unset($this->array[$offset]);
    }
}

$x = new Example();
$x[] = "abc";
$x[] = "def";
var_dump($x->array);
?>

Expected result:
----------------
// Using ArrayAccess as opposed to AppendableArrayAccess
// which doesn't currently exist
the array: array('def')

Actual result:
--------------
the array: array('abc', 'def')