WeakRef

I implemented a weak reference object in build 671.  Since it requires
notification of when an object is deleted, it was easy to add a feature to
allow a callback when the reference object is read, written or deleted.
This would be useful for debugging and maybe other situations.  The delete
callback is an external alternative to the del_ method also.

The methods are...

weakref = WeakRef(obj)
WeakRef.str_()   =>  <Prototype:WeakRef>
weakref.str_()     =>  <WeakRef:obj.str_()>
obj = weakref.ref()   # throws exception if obj doesn't exist anymore
weakref.exists?()

The notify feature has these constants to indicate whether you want
notification on a read access, write access, or deletion of the obj.

WeakRef.READ
WeakRef.WRITE
WeakRef.DELETE

The notify method sets up a callback (func) with an optional param for the
callback and flags to tell under which circumstances to do the callback.  If
you call this with flags = 0 then the callback will be removed.

WeakRef.notify(func, param=None, flags = WeakRef.WRITE | WeakRef.DELETE)
weakref.flags()

Comments?