Avoid spawning the async-io thread if possible

[Kestrer]

In most programs that use async-io, the user will simply run block_on at the top level. However, this will still spawn the async-io thread despite it not being necessary. Would it be possible to avoid that?

[notgull]

I don't know, this feels like it would be an easy footgun to accidentally set off. See the issues with #59

[taiki-e]

Yeah, both of the currently submitted PRs seem to cause problems with the existing code when the threads are disabled.

[notgull]

@taiki-e Do you mind if I close this as well as #59? At worst, all the async-io thread will do is idle on the reactor lock and occasionally pick up slack. Even in those cases, it can prevent programs from grinding to a halt. The only reason I can think of to plan this is if we wanted to port smol to no_std for some reason.

[taiki-e]

Yeah, theoretically it would be more efficient to have no threads, but I don't see this as some kind of bug or problem that needs to be fixed in the near future.

[notgull]

Reopening this as apparently this is a sticking point for some users