add basic defmt support with feature gate

[shulltronics]

Hi there,

Thanks for your awesome work on this crate :) I'm using this in a project and wanted to be able to debug via defmt-rtt so I've added the derive attributes to support that.

Let me know if there is anything you'd like changed, or if you don't want this feature at all (though I think it's useful for no_std work).