Add Async Recursion Macro to Workarounds

[zicklag]

This may not be something you want in the book, but there is a nifty macro crate for automatically transforming async functions to return boxed futures that could be useful to people.

I noticed the other example was in a separate file, but this is a super small example so I just inlined it. I can change it if you like.

[taiki-e]

I wonder if this is no longer necessary because impl trait recursion was allowed some time ago.

[zicklag]

Looks like a simple recursive async function still fails ( playground ).

[taiki-e]

Ah, thanks. And it is impressive that the compiler's diagnosis mentions async_recursion crate.

  = note: consider using the `async_recursion` crate: https://crates.io/crates/async_recursion