Add support for smol, deprecate async-std

[bikeshedder]

async-std was deprecated 5 months ago.

As of now deadpool-runtime only supports tokio and async-std for timeouts and spawning tasks.

This issue is only about adding support for timeouts and spawning tasks on a given runtime.

The usage of tokio::sync::Semaphore will not be changed as this is the only fair async semaphore implementation available at the moment:

• Add feature to use async-std synchronization primitives instead of the ones from tokio #88
• Fairness of semaphore smol-rs/async-lock#23