std::chrono hsTimer

Note that on Windows with Visual Studio 2013, the smallest guaranteed tick of the `std::chrono::high_resolution_clock` is 1µs, so it's possible that we've lost precision here. That's a known bug in VS that is fixed in the next version.

This definitely needs to be tested in actual gameplay, and it probably causes some conflicts with the std::thread/pnAsyncCore refactor (sorry @zrax)