Add defmt support and highlight crate features in docs.rs

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This adds support for defmt via the defmt feature flag. This mainly brings defmt to the error types, but also adds it to Orientation.

To improve documentation, I added a rustdoc conditional feature for highlighting gated feature. All of the defmt::Format implementations could have been simply derived with a cfg_attr, but it doesn't work smoothly with the #[cfg_attr(docsrs, doc(...))] annotation.

To test documentation, you can run:

RUSTDOCFLAGS='--cfg docsrs' cargo +nightly doc --open --all-features

It should render like this:

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Since you're using micromath, I added a PR for defmt support there as well (tarcieri/micromath#108). If that happens, I'll come back and forward the feature.