Update to async/await

[mre]

Async/await has landed. And with it, there were quite a few changes on the Rust ecosystem.
Our lovely dependabot wanted to update futures to 0.3, but it's not that easy. Once egg-mode-rs/egg-mode#73 (or similar) lands, we can give it another try.

[mre]

WIP branch at https://github.com/hello-rust/hello/tree/futures03