c.c rust-lang/rust-analyzer#19196

I'm afraid that implementing this on chalk and using on rust-analyzer would make currently ongoing rust-analyzer's trait solver migration more difficult but we need this to improve user experiences for Rust's shiny new async closures.

1. LangItem and WellKnownTrait for AsyncFn*
2. TyKind for async closures. rustc categorizes them to CoroutineClosure but rust-analyzer categorizes them to Closure. I'd like to follow the rustc way for consistency but this might hinder currently ongoing trait solver migration in rust-analyzer a bit
3. Built in AsyncFn* implementation supports for async closures and functions that returns future

I'm planning to start working on this tomorrow but if this is not welcomed due to next gen solver migrations or other issues, I'll gladly look for alternatives 😄