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

From: Daniel Frey (d.frey_at_[hidden])
Date: 2008-04-21 18:18:20

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