Using async when not required will increase compile times, and propagate async-ness to the callers, transitively.
See also the [lint description](https://rust-lang.github.io/rust-clippy/master/#/unused_async).
Since we only had a few false positives, I've enabled it by default.
The clippy website explains that `Box::new(Default::default())` can be
shortened to `Box::default()` and that it's more readable. That's true…
when you don't have a generic parameter in the mix (which may be
required if rustc can't infer the type of the boxee), in which case it
just looks bad, e.g. `Box::<MyType>::default()` instead of
`Box::new(MyType::default())`.
I do strongly prefer the latter, and propose to get rid of the lint, as
a result.
