Merge Boost.Variant from trunk
* documentation, tests and implementation of multivisitors (fixes #8459)

git-svn-id: http://svn.boost.org/svn/boost/branches/release@84282 b8fc166d-592f-0410-95f2-cb63ce0dd405
Spirit: merging from trunk

git-svn-id: http://svn.boost.org/svn/boost/branches/release@84286 b8fc166d-592f-0410-95f2-cb63ce0dd405