[boost] [smart_ptr] optimize current enable_shared_from_this with 100% compatibility

The attached patch optimizes the current implementation of enable_shared_from_this, it saves one plain pointer. That might not be very much, but it's a start. It is 100% compatible for the users, no features are dropped. Further improvements might be possible, but I think the attached patch is a good base. Peter, do you see any chance for it? If not, please tell me why and (if possible) what needs to be improved. Regards, Daniel