this is our first time using the Rust smol library, which seems quite nice. pleasingly, there isn't some big war between the authors of different rust async runtimes; rather, roughly the same group of authors wrote first tokio, then async-std, then most recently smol. this last one refactors the whole thing into a bunch of tiny, loosely-coupled libraries; smol itself is just a shorthand to import a few of those libraries at once. so that's pretty neat.