RFC: Future Combinators (`CompioFuture`)

[George-Miao]

Tracking Issue: #629

Add future combinators and FutureExt trait to compose different configurations. This gives the users the ability to furthur customize the not-yet-fired future in a builder manner, and call await on it after configuration:

let token = CancelToken::new();
let personality = /* */;
file.read(buf).with_cancel(&token).with_personality(personality).await;

Data will be passed through Context::ext, which passes an &mut dyn Any to any sub-future within a future.

[George-Miao]

One issue I'm facing with this design is that since compio-io uses async-fn-in-trait, we would have to convert any CompioFuture to Future with into_future first before returning, and some information will lost during the process (e.g., cancelability may only occurs on CompioFuture combinators, not the into-ed futures). We may choose to return CompioFuture in compio-io traits, or constrain CompioFuture to extend Future instead of IntoFuture.

[AsakuraMizu]

Can we make use of Context::ext to implement this? Although it's still behind a nightly feature...

[George-Miao]

@AsakuraMizu Yeah that's also a way