Add explicit support for arduino-nano

[drmorr0]

1. Add the arduino-nano feature flag to arduino-hal.
2. Make the ADC6 and ADC7 pins only available for nano, not for uno.
3. Add an example crate for arduino-nano with just a blink and ADC example.

[Rahix]

Hey, thanks a lot for the PR! I've got one thing I'd like to see differently: Instead of the "negative" opt-out feature I'd prefer a "positive" opt-in feature because this composes better with the way cargo merges features.

[Rahix]

In general, features should always be opt-in instead of opt-out, because features are selected based on the union of all features requested by downstream crates. I.e. suppose crate A sets disable-unused-adc while crate B expects the pins to be available - this wouldn't compile. If, OTOH you'd have an additional-adc-pins feature which crate A ignores and crate B sets explicitly - all is well.

This also means you can get rid of the not() you have to use everywhere.

[drmorr0]

yep, this makes sense, thanks!