[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).