Add simple register manipulation example for Uno boards

[burumdev]

Direct register manipulation can result in significant program binary size savings in Arduino IDE/C++ setups. With this example in avr-hal/rust, I aim to do two things:

1. To see if a similar amount of savings possible in avr-hal by opting to direct bit manipulation.
2. To serve as a simple quickstart in register manipulation for avr-hal users.

The result of the first item is that only token gains in program binary size can be achieved with direct register manipulation technique.