[experiment / RFC] runtime-agnostic async API

[japaric]

This PR contains an async adapter that works with both async-std and tokio but depends on neither crate. Instead it uses the futures::Async{Read,Write} interfaces to make the API, runtime generic.

• this is just a sketch (= I have not done extensive testing)
• the sketch only includes the client API
• a client example that can use either async-std or tokio is included
• there are no API docs but the API surface mimics the high level async API found in rustls-tokio
• the API is not no-std compatible because the Async{Read,Write} traits include the std::io::Error type in its interface
• the async API is implemented on top of the UnbufferedConnection API

[cpu]

@quininer If you had time this branch might be interesting to you as someone that has done a lot of work on rustls-tokio. Any feedback you could share would be helpful !

[quininer]

This looks great, I have plans to use it in tokio-rustls, futures-rustls after UnbufferedConnection is released.

The async crate has a lot of TODOs, but i think direction is fine.

[joshtriplett]

Suggestion: rather than using the full futures, consider using futures-io (https://crates.io/crates/futures-io), to reduce dependencies for people who don't need the full futures.