Auto merge of rust-lang#17258 - maxwase:code-assist-async-sugar, r=Veykril

Add `toggle_async_sugar` assist code action

Implement code action for sugaring and de-sugaring asynchronous functions.

This code action does not import `Future` trait when de-sugaring and does not touch function boby, I guess this can be implemented later if needed. This action also does not take into consideration other bounds because IMO it's usually "let me try to use sugared version here".

Feel free to request changes, that's my first code action implementation 😄

Closes rust-lang#17010
Relates to rust-lang#16195

Author: bors