Add unawaited_futures to lints

[Alvarenga1]

Is there any reason why the rule unawaited_futures was removed ?

From what I understand not awating for a future is considered an error, and we should use unawaited for such cases.
Not waiting for a future to complete can cause all sorts of issues and sometimes it's not immediately obvious that a future is not being awaited, I think it would be a good idea to have this rule added back to the lints.

[devoncarew]

Ah, there was discussion about unawaited_futures here: #25 (closing this issues as a dup).