Update async-std and stop-token dependencies, migrate to stable channels

[seijikun]

Updated async-std and stop-token dependencies.
The new async-std stabilized the channel api, and relocated them, so I migrated from async_std::sync to the new namespace async_std::channel.

The send() methods in the new stable channel api now returns Results. I didn't want to assume what to best do with these, so I added TODOs.

[flub]

Thanks! Let's clean up the TODOs and then it's good to merge i think.

[yancyribbens]

Will this fix be available on Crates.io? Currently 0.4.1 on crates.io fails to compile, and so cargo must be pointed to this git repo.