Drop async_trait in favor of native async traits

[olanod]

async_fn_in_trait has been stabilized, declaring the crate's traits as "native" async traits can bring some performance benefits or make the code simpler, it could also help to towards a no-alloc version of the crate that works better in embedded devices as futures no longer have to be Boxed.

[niklasad1]

Yes, we are very exited about it but it's not released in rustc stable yet and last I heard, it's planned to be in the rustc December release.

Once it's in rustc stable we will get rid of the async_trait.
Would also be nice to get rid off the extra async_trait annotations as well as the Boxed futures.

[olanod]

I think it's worth introducing the the change early as code that uses nightly can already benefit(e.g. runtimes) and the feature will likely be in stable by the time a relevant PR gets merged.

[bkchr]

I think it's worth introducing the the change early as code that uses nightly can already benefit(e.g. runtimes) and the feature will likely be in stable by the time a relevant PR gets merged.

We don't use nightly anymore for runtimes and we also don't use async in the runtime. So, no need to "rush". It will come with rust 1.75.0 at 28.12.2023.