Make async-std dependency optional

[akhilles]

Hello! Would it be possible to make the async-std dependency optional and feature-gated? Currently, async-std is used for the following: async_std::io traits, async_std::sync::channel, and async_std::fs. If futures-lite and async-channel are used instead (NOTE: these are already dependencies for async-std, so no new dependencies would be added), a bunch of transitive dependencies would be removed without sacrificing any functionality.

Cargo.lock diff, with default-features = false, after making async-std optional: https://gist.github.com/akhilles/60d553d797f9e0e94056da3274285e31.
It's also a fairly straight-forward change: https://gist.github.com/akhilles/29a8de956cf7beb274a2f8d6959d8e4f

Feel free to close this if it doesn't align with this library's design goals.

[yoshuawuyts]

Hi, sure we can do this — but in order to not break the API we should enable it by default. A PR to add an async-std feature flag would be welcome!