Add nrf softdevice feature

[nospam2678]

As has attempted to be described in the commit message, this is intended to address using this crate with Nordic Semiconductor's SoCs by adding a feature for acting slightly different with them. Since it adds a dependency on embassy, which is not yet distributed through crates.io, this is a draft pull request until the required crates have been stabilized.

There are likely improvements to be made to the details, but the main question should likely be; Could this addition stand a chance to get merged once the eco-system matures, or would doing this in some other way be considered a more appropriate solution?

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @adamgreig (or someone else) soon.

Please see the contribution instructions for more information.

[Dirbaio]

I think the right fix is to use critical-section. nrf-softdevice provides a critical section implementation suitable for the weird softdevice interrupt handling (currently for CS 0.2, this PR updates it to 1.0).

This way this crate works with all kinds of CS implementations without having to add them here.

See #56

[nospam2678]

#56 sure looks like the better solution. I'm closing this attempt.