Added swap to signal classes.  Fixes #8449



git-svn-id: http://svn.boost.org/svn/boost/trunk@84006 b8fc166d-592f-0410-95f2-cb63ce0dd405
Tweaked template declaration of swap for signal classes, to make
it more consistent.



git-svn-id: http://svn.boost.org/svn/boost/trunk@84007 b8fc166d-592f-0410-95f2-cb63ce0dd405