Switch to Tokio from async-std

[nyabinary]

Looking at async-std commit history, it's practically a dead project, so I think it's wise to start migrating the codebase to tokio.

[ranfdev]

async-std integrates pretty well with gtk-rs and it's glib MainLoop. To use Tokio I would have to manually start a Tokio runtime. I'm not sure how the Tokio runtime and glib's mainloop would interoperate.

Maybe I could even ditch async-std completely and try using only glib... I have to see...