Remove dev dependency on async-std

[paulhowardarm]

Summary

This crate currently has a (dev) dependency on async-std, which has been deprecated. We should use smol instead.

Details

async-std is unmaintained and smol is the stated alternative. This was picked up through the advisories mechanism in cargo-deny in the CI build.

A sticking plaster was applied to ignore the advisory in #29 to avoid holding up the merging of the PR. Since it's "only" a dev dependency, it's not that critical.

Once this issue is addressed, we should remove the entry in deny.toml where the advisory is ignored.

The advisory in question is RUSTSEC-2025-0052.