Add CI job to check compilation works with tokio

[sveitser]

I think running cargo check is enough here.

env RUSTFLAGS='--cfg async_executor_impl="tokio" --cfg async_channel_impl="tokio"' cargo check

Not a high priority now because we'll make it less easy to mess up the RUSTFLAGS.

[sveitser]

Comment from @jbearer: ... if we want to run with tokio, we need to replace all our async_std::task_spawn with async_spawn from the compatibility layer, and similarly with all of our other explicit async_std references.