For swap use ADL instead of specializing std::swap

[randombit]

Not using ADL is a remnant of pre-C++11

[reneme]

Isn't that (technically) a breaking API change that requires users to switch from std::swap() to just swap()?

At least that's how understand the example code in the STL documentation.

[randombit]

I don't think this is a breaking change. It does mean that using std::swap, at least with the usual implementation of

T c = move(a);
a = move(b);
b = move(c);

will be a bit slower than it was since we do 3 swaps instead of 1. But it still works just fine.

I'm basing this change on comments from Dave Abrahams and Howard Hinnant in this SO post https://stackoverflow.com/questions/11562/how-to-overload-stdswap