Description:
------------
It would be nice to be able to create factory constructors (like Dart offers). 
This could be achieved by allowing __construct to return an arbitrary object of 
the same class or a descendant thereof.

No return statement would then mean "return $this;" as it is today.

Sure, one can use a static factory method, but it's not as elegant and cannot 
simply be added to existing code without modifying all places where objects are 
instantiated.

Test script:
---------------
class MyClass
{
  public function __construct($id)
  {
    if (isset(self::$cache[$id]))
      return self::$cache[$id];

    // Do expensive setup

    self::$cache[$id] = $this;
  }

  private static $cache = array();
}