Arcade Drive implementation

[cyndiac]

In order of priority & difficulty:

• invert direction of one side of drive motors so that fwd input results in forward driving on both sides
• implement arcade drive function
• add slow mode (boolean that applies a speed multiplier, ~0.4-0.7 depending on driver preference)
• add speed ramping function (if you save the previous loop's inputs, you can limit the next loop's change in velocity)
• coast mode during teleop
• brake mode upon disable