Add impls for the Defmt logging crate

[BenFordTytherington]

• Add impls for the main structs where it is obvious how to do so
• Gate this behind a new cargo feature called "defmt"

[djc]

I am not excited about adding these. Unfortunately coherence is a bit of a challenge especially in cases like these where (a) neither crate is clearly more "high-level" than the other and (b) neither crate has a stable (1.0+) API. At the very least, I think we'd want this to be a defmt-03 feature.

[BenFordTytherington]

From Defmts perspective, implementing the trait here is a better solution due to being able to access all the private fields. Additionally, defmt 1.0 is planned to be released early next year, with hopefully no breaking changes compared to the current version.

[djc]

From Defmts perspective, implementing the trait here is a better solution due to being able to access all the private fields. Additionally, defmt 1.0 is planned to be released early next year, with hopefully no breaking changes compared to the current version.

Well, if that's the case I'm going to want to hold off until defmt 1 has been released.

[jonathanpallant]

defmt-1.0 is almost here! We're publishing 1.0-rc1 very shortly.

But, we're also going to publish a 0.3.100 which does the semver-trick to re-export defmt-1.0 as-is. So any crate using defmt-0.3 will get a free upgrade to defmt-1.0 with no changes required.