[Bug] .into_async_callback() fails for systems with only the In parameter

[mxgrey]

The following example results in a compilation error:

async fn page_title_callback(In(url): In<String>) -> Option<String> {
    let response = trpl::get(&url).await;
    let response_text = response.text().await;
    trpl::Html::parse(&response_text)
        .select_first("title")
        .map(|title| title.inner_html())
}

let title_callback = page_title_callback.into_async_callback();

even though that same function can be made into an async service. It can even be made into a callback if AsyncCallbackInput is used instead of In.

It's not clear to me where the incompatibility is coming from, but I would guess it's related to the additional Task generic in the IntoAsyncCallback trait.

This isn't a crucial error because this type of function can just be made into a map instead. There's no reason for this to be a Bevy system. Adding even one more system parameter of any kind to the function allows .into_async_callback() to work.

Still it's an odd limitation, and worth looking into if anyone has spare time and interest.

[mxgrey]

Closed by #177

.into_async_callback() is no longer needed whatsoever.