- Renamed/ordered the directories for macros, clippy, and conversions, shifting them all one up in numerical order to fit async as number 21
- Added the 21_async directories in exercises and solutions
- Added async1.rs in the 21_async directory in exercises and solutions
- Added async to the info.toml in rustlings-macros and reordered the rest of the exercises in there as well
- Created an exercise for async1 and its solution