[boost] [variant] Heads-up!

Hi, I've merged a very cool optimization by Nikita Kniazev into the master branch. From now on boost::variant does pointer stealing for recursive variants. This significantly improves the performance of the variants move constructors. However if you use a variant variable after the std::move for anything except destruction and assignment then you're getting an UB. Beware! -- Best regards, Antony Polukhin