Consider dropping async-std support

[CosmicHorrorDev]

A long time ago in a distant thread there was the proposal to drop async-std support in favor of the existing tokio runtime. The conclusion at the time was to continue supporting async-std

Now many moons have passed and aysnc-std (1.13.1) now decrees

async-std has been discontinued; use smol instead

and who are we to say otherwise?