mpark::detail::impl::swap not marked with noexcept

[eyalroz]

Typically, swap() functions should be noexcept. Yet - mpark::detail::impl::swap() is not marked noexcept - nor is it marked noexcept(false), nor any other way. It should be marked, and perhaps even actually marked noexcept if that's merited.

Note: I've noticed issue #2 which regard this matter to some extent, but the code seems to have changed a lot since that discussion, and in the PR I don't see an impl::swap().