[Vc] Deal with the missing swap() on Vc proxies.
This is a problem with Vc 1.3.0. Vc 1.3.2 seems to have implemented
swap()
.
We work around it using copying and a temporary to implement swap()
on our
own wrapping proxies. Any member type of a Vc SIMD type should be a
fundamental type, so we don't bother using moves.
Closes: #59 (closed).