Async optimizations

[insanitybit]

Some async performance stuff:

1. Concurrency where possible ie:

don't do:

let a = foo().await;
let b = bar().await;

do:

let (a, b) = join!(foo, bar).await;

2. Also, spawn may not be ideal for performance. It can be better to refactor to avoid that.

3. Recursive futures imply a heap allocation for every frame. If you can refactor the code to be imperative you may be able to avoid that.

Just some thoughts!

[frederikhors]

Wow, can you create a PR with examples @insanitybit? Amazing suggestions!