Use `AsyncFn` when targetting Rust 1.85+

[Tpt]

The AsyncFn trait has been introduced in Rust 1.85. It might be handy to use it in TwirpRouter::route and GrpcRouter::route methods

[vsiles]

afaict that trait is still unstable. Do we want to release 0.3 without this ?

[surajk-m]

Hi @vsiles @Tpt,

For now, we should rely on stable features:

F: Future<Output = Result<O, TwirpError>> + Send + 'static,
C: Fn(S, I, RequestParts, RS) -> F + Clone + Send + Sync + 'static,

I recommend proceeding with these changes for the 0.3 release. Once AsyncFn stabilizes in a future Rust version, we can consider adopting it in a later release.

[Tpt]

Thank you @surajk-m! We will do so