* Fix a test.
* Started implementing Direction#next.

Direction will know the next (to the right) direction.
* Finished Direction#next.
* Refactor test.
* Added Direction#prev.

Going counter clock-wise. E.g. Robot::Direction::NORTH.prev =>
Robot::Direction::WEST.
* Refactor tests.
* Add Direction#next! and Direction#prev!.

Replace a direction with next (clockwise) or prevous (counter clockwise).