Allow 1-square vehicles to be drivable if they have stable wheels

Closes #3261, #3384.

If a vehicle is only 1 square big, and has a wheel with the "STABLE" flag, it is considered to have 'enough' wheels.

Also, unicycles. The use bicycle wheels, but are installed as unicycle wheels (representing the extra work you need to do to have the crank directly drive the wheel, on a wheel the same size as a bike wheel).