Implement std::exchange per N3668

[Howard Hinnant]

On Jul 8, 2013, at 4:08 PM, Marshall Clow <mclow.lists at gmail.com> wrote:

> 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
> 
> <n3668.patch>


Looks perfect!  Please commit, thanks!

Howard