remove async_await feature gate

[yoshuawuyts]

Removes the async_await feature gate, due to land in tomorrow's nightly (ref rust-lang/rust#63209 (comment)). Thanks!

Code used to remove feature gate

#!/bin/sh

files="$(fd .rs)"

for f in $files; do
  sed -i '/async_await/d' "$f"
done

cargo fmt -- --edition 2018

[yoshuawuyts]

Tests pass, I think this should be good to merge now!

[kpp]

ping?

[skade]

I think this will fail on docs.rs...

[skade]

It will not fail on docs.rs anymore :)

[skade]

We also need to go through a-chat.

[skade]

I'm going to merge this for release prep.