Add keyboard peripheral example with builder

[dzervas]

No description provided.

[alexmoon]

Thanks! nrf-softdevice now supports rustc stable, so we can't use any nightly features. Is it possible to rework your hid macro to use macro_rules! instead?

[dzervas]

oh right! macro_rules!, I completely forgot about it!

[alexmoon]

Thanks! Can you also revert the formatting changes to Cargo.toml?

[alexmoon]

I think you still need your [[bin]] section? I was just asking you to revert the formatting changes that were unrelated to this PR.

[dzervas]

oops, reverted a bit too much!