Macro async_executor

macro_rules! async_executor {
    () => { ... };
    (if { $($p:tt)* } else { $($n:tt)* }) => { ... };
    ($($p:tt)*) => { ... };
}

Indicates async_executor is used as the future execution backend.

This macro passes the provided code because #[cfg(feature = "async_executor")] is currently active.