This acts an an initial port to async/await using the smol runtime. The only parts of the codebase that still use regular threads and blocking IO should be the webcam ALS and the wayland capturer, as the v4l and wayland crates don't yet have support for async IO. No sleep loops have been removed yet, but this should make it easier to do so as we've discussed in #143.

Work done

• Replaced the threads spawned "green threads" using smol::spawn
  • The 2 places this isn't done yet use smol's thread pool using smol::unblock
• Ported IO operations to non-blocking variants (please check if I've missed any!)
  • The config reading code hasn't been ported (yet), it shouldn't be significant as it only does a little work on startup.
• Replaced std::mpsc channels with smol channels.
• Converted several traits to enums. Traits with async functions are a problem, because the return type of these functions are some compiler-generated future type that's different for each implementation of the type. This causes the problem that the trait would have to be generic over this type, preventing us from Box-ing it.
  • As a consequence of this, mockall can no longer generate a mock implementation for Brightness. Tests that use mocking have been commented out for now. I think this might be best left to another PR.
• The main function as well as tests have been given a macro to allow them to be async.
• Several pieces of code that previously used "monadic" functions with Result/Option has been rewritten with plain logic as the closures given to these functions cannot support async code.
• std::sync::Mutex has been replaced with smol::lock::Mutex
• A type alias ErrorBox for boxed errors has been created as the type got a few more trait bounds now. We already transitively depend on anyhow, perhaps we should use that for error handling in the future.
• Some clippy lints have been fixed on the way, mostly assert_eq!(true, x) -> assert!(x)
• Maybe other stuff I've missed :D

All tests pass and wluma has been tested with the IIO sensor ALS on wayland/River.