Consider switching to Axum #130

I've had some issues with Actix (see: #81, and actix-web issue 1913). I've used Axum and don't remember running into those issues. (Though, TBF, that was a different use case, so the issues might exist there too.)

Would it be worth switching to Axum?

This will probably be simpler after the Diskuto refactor, when the surface area will be greatly reduced.