Macro async_backtrace::frame

macro_rules! frame {
    ($async_expr:expr) => { ... };
}

Include the annotated async expression in backtraces and taskdumps.

This, for instance:

tokio::spawn(async_backtrace::frame!(async {
    foo().await;
    bar().await;
})).await;

…expands, roughly, to:

tokio::spawn(async_backtrace::location!().frame(async {
    foo().await;
    bar().await;
})).await;