Add "rest" and "active" positions to Servo

[Gidntsquia]

Every servo has two main positions: the position it starts in and the position it moves to in order to do an action. We can standardize this by adding "rest" and "active" position constants to the Servo class along with corresponding functions like move_to_rest() and move_to_active(). The "rest" position would be where the servo starts and the "active" position would be where it moves to do something. A claw's "rest" could be open and "active" closed.