Further Class::Struct patching from Damian.

Further Class::Struct patching from Damian.

The new new semantics are:

* If you pass an initializer for a nested object, it has to be:
	a) a reference to an object of that class, or
	b) a reference to an object of a subclass of that class, or
	c) a reference to a hash of arguments to be passed to the
	   class's constructor
* If you pass no initializer for a nested object, the attribute
  is initialized to C<undef>

p4raw-id: //depot/perl@11976