Replace cap-async-std

cap-async-std has been removed without a replacement (see https://github.com/bytecodealliance/cap-std/pull/399), because async-std is no longer maintained.
Now there is no way of using async code with this crate.

It would make sense to add either a:
- cap-tokio (I would prefer this one)

or a
- cap-smol

crate to this repository to retain the async functionality of this project.