Arduino UNO Example

[JohnDoneth]

Now that Rust has nightly support for AVR it would be neat to have an example for this crate using avr-hal for the ubiquitous Arduino UNO.

[ColinTimBarndt]

In #54, I added an example for the Arduino Nano, but it's general enough to work on any Arduino as it's using arduino-hal, which is part of the same project and depends on avr-hal.