Add feature flag for defmt derives for errors

[thelenlucas]

Added a new new non-default feature flag defmt, adds an optional dependency for defmt, a lightweight logging crate for embedded machines, allowing for debugging over RTT.

I dislike mutually exclusive features, but it might be worth adding here, as the std error's do not implement defmt::Format, as defmt is not intended to be used with the standard library. As such, they are wrapped with debug, which is inefficient, but unlikely to occur, due to the prior point. If we really wanted to break it out though, it should be trivial to do so via compile_error!.