Please add support for embedded-hal-async

[quentinmit]

The I2c trait from embedded-hal-async would allow this crate's usage with embassy.

Unfortunately, I can only think of hacky solutions to support both blocking and async:

• Make a second copy of all the functions, doing the rote fn -> async fn transformation and sprinkling in some .await calls
• Macro magic to define two copies using the same source

and then another question is:

• Keep the same type names and swap in async using a feature, or
• Expose the methods on a different type or in a different module (still feature-gated).

I asked in rust-embedded/embedded-hal#356 if there's any best practice here. I think most of the other crates I've seen have gone for some form of duplicate code.