Add optional support for printing some types with defmt

[mattico]

defmt ("de format", short for "deferred formatting") is a highly efficient logging framework that targets resource-constrained devices, like microcontrollers.

https://defmt.ferrous-systems.com/

I've been maintaining this support in my fork for some time, it hasn't been burdensome. defmt has become popular enough that I think it is worthwhile to upstream this. Supporting defmt formatting in this library makes it easier to print error messages (don't need to use defmt::Debug2Format) and eliminates the code bloat of the core::fmt machinery. We could similarly move Debug derives behind a feature flag to help people who are trying to eliminate core::fmt, but I don't particularly care.

[mattico]

Okay, defmt 0.3 crates require edition 2021 - MSRV 1.56.0. I could bump the MSRV, or just change CI to not use defmt 0.3 on 1.52.0.

[richardeoin]

Thanks, this is quite nice (although I've not used defmt much myself). Could you add a short section in the lib.rs docs to help users find this flag? Otherwise LGTM!

I bumped the MSRV in #236 so should be good now

bors delegate=mattico

[bors]

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[mattico]

@bors r+

[mattico]

@bors r+

[mattico]

bors r+

[bors]

Build succeeded:

• ci (1.56.1, stm32h735)
• ci (1.56.1, stm32h743)
• ci (1.56.1, stm32h743v)
• ci (1.56.1, stm32h747cm7)
• ci (1.56.1, stm32h753)
• ci (1.56.1, stm32h753v)
• ci (1.56.1, stm32h7b0)
• ci (1.56.1, stm32h7b3)
• ci (stable, stm32h735)
• ci (stable, stm32h743)
• ci (stable, stm32h743v)
• ci (stable, stm32h747cm7)
• ci (stable, stm32h753)
• ci (stable, stm32h753v)
• ci (stable, stm32h7b0)
• ci (stable, stm32h7b3)
• Rustfmt