Implement std::exchange per N3668

[Marshall Clow]

Reference: http://wiki.edg.com/twiki/pub/Wg21bristol/FormalMotions/N3668.html

Another small patch ;-)

-- Marshall

Marshall Clow     Idio Software   <mailto:mclow.lists at gmail.com>

A.D. 1517: Martin Luther nails his 95 Theses to the church door and is promptly moderated down to (-1, Flamebait).
       -- Yu Suzuki

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