Please make embedded-devices-derive a seperate crate

[Tupelov]

I think the project structure should more closely emulate embasssy's which has multiple crates within the same git repo, the reasoning for this is that it makes it easier to use embedded-devices-derive independently, currently you need to add async cfg and an async feature to your project if you want to make use derive without including embedded-devices

[oddlama]

I've restuctured the crate recently, this should now be using the layout you requested.