Revamping the book to focus on usage

[betamos]

Following some conversation in wg-async-foundations, we've been proposing to revamp the async book to focus better on using and understanding async Rust. Currently, the book mixes advanced topics such as building an executor, understanding wakers and implementing Future manually, with fundamental knowledge required to properly understand and use async Rust in order to Build Stuff™. I believe the book can become a more useful central resource with some structural changes.

The need for more comprehensive async documentation arose when we at Fuchsia have realized (and in fact, measured) that people struggle with the async programming model more than any other area in Rust (even including complex, under-documented internal crates).

The current state of async Rust is fragmented into different runtime eco-systems. Our initial thoughts were to build yet-another set of docs for Fuchsia devs, but I hope we can do better in bringing shared knowledge to a central place. Here's a draft of what we have so far. So far, the reception has been positive, but we need more concrete feedback.

Is this something that sounds interesting? I'd be happy to drive this, and can also help find reviewers if need be. Also, feel free to reach out in the wg-async-foundations topic linked above.