Support async/await in Mojo::EventEmitter

[kraih]

Currently using Mojo::EventEmitter with async/await is very awkward, since the promises returned by async functions need to be referenced manually. This PR is a possible solution, but there is a performance cost. We use Mojo::EventEmitter very extensively in Mojolicious (and countless 3rd party modules), so it is hard to say if this could become a real problem.

Here are before and after example apps.

[kraih]

I'm very unsure what the correct way for handling this is. The finally hack could also be applied at the Mojo::Promise async protocol level, but that essentially disables a feature of Future::AsyncAwait to detect unhandled promises.

[kraih]

I'm bringing this up so early because i'm fairly certain many of our users will want to use async/await with WebSockets.

[kraih]

After thinking some more about this, the impact on the Mojo::IOLoop::Stream layer really worries me. Some events are very relevant to overall performance.

[kraih]

I think we can rule out this solution, it's pretty terrible.