Drop async-trait

[matze]

This is a preview and in anticipation of Rust 1.75. But it works well with current nightly either. Note that the public trait API requires the desugared form in order to specify the Send bound on the returned future.

[qdot]

There's gonna be an interesting question about how far back w/ rust versions we want the library to be compatible, so I'm hesitant to merge this right after the 1.75 release (currently scheduled for 2023/12/28). That said, I'll definitely let this hang out and am looking forward to the feature going stable.

[matze]

One option would be to make the async_trait macro usage be a default feature (for the foreseeable future) and when opted out switch to AFIT. But I can imagine that this looks rather ugly with lots of #[cfg(…)]s