Implement swap.

Nobody has claimed this yet.

In the current code there is no implementation for `swap()` which means `std::swap()` will be used. This should do the right thing (as long as the move constructor and move assignment do the right thing) and the [tests](https://github.com/mapbox/variant/blob/861faa8125ae7c2d9e41e33b3d5d97a17213c415/test/t/variant.cpp#L268-L300) show this.

But this solution is not optimal, because the default `std::swap()` will always do one move construction and two move assignments (in this case of the variant). If there is a more efficient `swap` for the type in the variant, it will not be used. The upcoming standard [P0088R0](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0088r0.pdf) describes the effects of swap as: "if `index() == rhs.index()`, calls `swap(get(*this), get(hrs))` with `i` being `index()`. Else calls `swap(*this, hrs)`."

We might want to implement our own `swap` in the future.

Start by reading the existing variant move and swap-related code, then run the tests in test/t/variant.cpp around lines 268-300. Compare the current behavior with the issue's same-index and different-index swap requirements. Done means the tests pass and swap can use the contained type's more efficient operation when appropriate.

Written by the indexing model from the issue text.