Add unused_async_trait_impl lint

[Wassasin]

changelog: [unused_async_trait_impl]: added lint that checks for trait impl functions that are unnecessarily async. By rewriting them to return a core::future::ready an unnecessary coroutine is prevented.

In an embedded context having many trivial async blocks potentially hurts code size significantly. This lint enables projects to quickly find the cases that this applies to.

Unsure about if this is the right method to replace the relevant code spans. Any feedback is very welcome.

[rustbot]

r? @dswij

rustbot has assigned @dswij.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer