[boost] none_t in the boost namespace

Hi people, In the users mailing list, Yuval Ronen pointed out that optional<> uses a type "detail::none_t", documented as such, thus prompting people to use a type in a namesapece which is supposed to be hidden to end users. A straightforward fix is to just move "none_t" out of the detail namespace directlty into boost. However, this little change introduces a new type in the (main) boost namespace, so before I do that I want to make sure it won't have any undesired impact. If you see any reason for not doing this (putting "none_t" right there in boost) please speak now. If no one objects, I'll do it. Best, Fernando Cacciola