it's been a while since I have touched `async`.  Searching isn't much help here, are these warnings at all relevant for embedded?

```
warning: use of `async fn` in public traits is discouraged as auto trait bounds cannot be specified
    --> src/lib.rs:1034:5
     |
1034 |     async fn read_regs(&mut self, reg: u8, buf: &mut [u8]) -> Result<(), Self::Error>;
     |     ^^^^^
     |
     = note: you can suppress this lint if you plan to use the trait only in your own code, or do not care about auto traits like `Send` on the `Future`
     = note: `#[warn(async_fn_in_trait)]` on by default
help: you can alternatively desugar to a normal `fn` that returns `impl Future` and add any desired bounds such as `Send`, but these cannot be relaxed without a breaking API change
     |
1034 -     async fn read_regs(&mut self, reg: u8, buf: &mut [u8]) -> Result<(), Self::Error>;
1034 +     fn read_regs(&mut self, reg: u8, buf: &mut [u8]) -> impl std::future::Future<Output = Result<(), Self::Error>> + Send;
     |
```