Use defmt for embedded debugging

[kiranshila]

Hey! First off, I love this library, takes a lot of the legwork out of writing device drivers.

I noticed that the debug printing interface is not no_std. It would be great if it was, so we can quickly check registers we read and write on microcontrollers, etc. It may make sense to pull in defmt or similar so we can get fancy printing without the additional bloat.