async-trait should add `must_use` declaration

[matthauck]

async-trait should add #[must_use] declarations on async trait functions. This compiler warning is rather important -- pretty easy to forget to call .await and then have a function call not actually execute that looks like it should be executing.

[matthauck]

Repro: https://gist.github.com/matthauck/9743152133266c50d5593a90867b34c1

I would have expected both do_trait and do_direct calls to fail with a warning.

[dtolnay]

Relevant upstream feature request: rust-lang/rust#67387