Feature: defmt support

[taks]

No description provided.

[robin-nitrokey]

Thanks for the PR! I have two requests:

• Ideally features should be additive. Currently, enabling defmt disables delog. Do you see a way to support defmt and delog independent of each other? Maybe with a custom macro that dispatches to defmt and/or delog based on the features?
• To avoid the duplication in the Debug and defmt::Format implementation for Error, I think it would make sense to have a pub const fn as_str(&self) -> Option<&'static str> that is used by both.

[taks]

Thank you for your review.

Strings written within the defmt::Format are not included in the logging data.
https://defmt.ferrous-systems.com/#operating-principle

However, if I move the strings to another function, the strings are included in the logging data.
For that reason, it is not a good to refactor into a common function.

By defining Error as an enum, we can use #[derive(Debug)].
If you'd like, I can redefine Error as an enum.

[robin-nitrokey]

I see, thanks for the explanation. Please add a short comment with what you just wrote to the defmt::Format implementation so that we don’t end up refactoring this in the future.

We deliberately don’t have Error as an enum so that adding new error variants is not a breaking change.

[robin-nitrokey]

I think you ended up using debug! instead of debug_lib! in some lines.