Added 2 utility metafunctions for variants. Necessary to make some algos variant aware.

git-svn-id: http://svn.boost.org/svn/boost/trunk@86584 b8fc166d-592f-0410-95f2-cb63ce0dd405
Tests for variant aware envelope.

git-svn-id: http://svn.boost.org/svn/boost/trunk@86585 b8fc166d-592f-0410-95f2-cb63ce0dd405