RFC: Future Combinators (`CompioFuture`)

[George-Miao]

Introduce experimental CompioFuture, which extends IntoFuture and adds some extra combinators. At its core, is a trait that looks like:

pub trait CompioFuture: IntoFuture {
    // combinator functions
}

with default implementation:

impl<F: IntoFuture> CompioFuture for F {}

And compio functions will return impl CompioFuture instead of anonymous futures generated by async. 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;

and this will not constitute a breaking change, as CompioFuture extends Future (via IntoFuture), and all current use cases should be a subset of what CompioFuture can do.

In combination with try_as_dyn, it's possible to write specialized implementation (especially for user-initiated cancellation, which will be in another RPC based on this).

[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.