Remove dependency on async_std (or make it optional)

[Ekleog]

Hello!

I've been starting to use duplexify for tests quite a lot. It works really well :)

This being said, I've noticed that the whole async-std appears to get pulled in when simply grabbing duplexify.

Do you think it'd be possible to make the dependency on async_std optional, or maybe to remove it altogether to simply depend on futures? I can try to have a look at contributing such a change if you'd be interested in it and don't have the time to look into it, though I can't promise anything about timeliness myself :)

Thank you anyway for duplexify!
Leo