big merge from trunk

git-svn-id: http://svn.boost.org/svn/boost/branches/release@85668 b8fc166d-592f-0410-95f2-cb63ce0dd405
big merge from trunk

git-svn-id: http://svn.boost.org/svn/boost/branches/release@85669 b8fc166d-592f-0410-95f2-cb63ce0dd405
Merge multiprecision changes from Trunk.

git-svn-id: http://svn.boost.org/svn/boost/branches/release@85681 b8fc166d-592f-0410-95f2-cb63ce0dd405