[boost] [variant] Applying a portion of fixes

Hi, I'd like to fix this issue: https://svn.boost.org/trac/boost/ticket/8772 Apply patch for this issue: https://svn.boost.org/trac/boost/ticket/7911 And take care of this: https://svn.boost.org/trac/boost/ticket/8717 Patches and ideas for fixes are described in tickets. If there'll be no objections till next weekend I'll commit the fixes. -- Best regards, Antony Polukhin