🗑 async-dropper-derive

`async-dropper` is probably the least-worst ad-hoc `AsyncDrop` implementation you've seen, and it works in two ways: - `async_dropper::simple` is stolen nearly verbatim from [this StackOverflow answer](https://stackoverflow.com/a/75584109) (thanks to [`paholg`](https://stackoverflow.com/users/2977291/paholg)!) - `async_dropper::derive` provides a trait called `AsyncDrop` and corresponding [derive macro][rust-derive-macro], which try to use `Default` and `PartialEq` to determine when to async drop. The code in this crate powers `async_dropper::derive`. See the `async_dropper` crate for more details.