Enhancement: Write "async_handler" proc_macro

[ZialeHub]

Added to pulsync-derive as a #[proc_macro_attribute]. It rewrites a plain async fn run into the dyn-compatible Pin<Box> form:

  // User writes:
  #[async_handler]
  impl AsyncTaskHandler for MyTask {
      async fn run(&self) {
          do_work().await;
      }
  }

  // Macro emits:
  impl AsyncTaskHandler for MyTask {
      fn run(&self) -> Pin<Box<dyn Future<Output = ()> + Send + '_ + Sync>> {
          Box::pin(async move { do_work().await; })
      }
  }

This reduce the complex type return for the user.