Could add a subcrate that only provide the `select!`?

[goldwind-ting]

Is your feature request related to a problem? Please describe.
Maybe someone wants to use select! in other runtimes, and the select macro is unrelated to runtime.

[Darksonn]

I don't want to maintain yet another crate just for the select! macro, but Tokio is released under MIT so there is no problem with you copying it into your own custom crate.