fix: Make full use of the tokio ecosystem if the tokio feature is enabled on Unix

[DataTriny]

I noticed that we can get rid of a couple of async-related crates when the tokio feature is enabled, because tokio ships its own async types.

[mwcampbell]

I noticed that the following line was removed in the select loop:

complete => return Ok(()),

I'm not sure I understand what that line did. Was it not needed in the first place, or is there some other change that makes it unnecessary even for async-io?

[DataTriny]

In practice this line is never executed since the app will terminate before both streams close. Since tokio's version of select doesn't allow this "complete" keyword, I decided to remove it. I think we could put it back if you want, with #[cfg(not(feature = "tokio))].

[mwcampbell]

If it's never needed in practice, I'm fine with leaving it out.