Add feature-gated support for defmt::Format

[ShakenCodes]

Supports defmt::Format operating with the types in this driver. This is especially useful for outputting Errors. To enable operating with defmt, set the feature "defmt" in the dependency.

[coveralls]

coverage: 88.71% (-0.09%) from 88.8%
when pulling 38a38c0 on Radiator-Labs:master
into 3a222c4 on eldruin:master.

[eldruin]

Sounds good to me, thank you!
Could you also:

• Rename the feature defmt-03
• Add an entry to the changelog
• Document this feature in the readme and lib.rs?
  This repo has en example.

[ShakenCodes]

Changes have been made. I chose not to add doctest examples in lib.rs, as I do not know how to enable the feature within the doctest context. Since usage is basically like anything else that derives debug or format, this is a pretty trivial usage. Hopefully the example in the readme is sufficient.

[eldruin]

Thank you!
I would also just add a couple of lines about the defmt-03 feature to the lib.rs, without examples.

[eldruin]

I would shorten this section to just this.

[ShakenCodes]

Are you suggesting deleting the two previous paragraphs?

[ShakenCodes]

Do we still want to add something to lib.rs? I could just move or duplicate the usage example from README.md

[eldruin]

yes, I would remove the introduction and example and just make "defmt" into a link to the crates.io page.
Then we are never outdated. Furthermore, I think people will most probably either already know about defmt or not care much about it.

[eldruin]

Thank you!