I left "free speed" alone since that's the technical term for it. In
general, velocity is a vector quantity, and speed is a magnitude (i.e.,
a strictly positive value).
This PR also replaces the speed verbiage in MotorController with duty
cycle.
Fixes#8423.
This does not deprecate any current functionality, but prepares the way for future deprecation.
The drive classes now accept void(double) functions, which makes them more flexible.
The C++ API ended up a bit more verbose, but the Java API is really concise with method references, which is >80% of our userbase. For example:
`DifferentialDrive drive = new DifferentialDrive(m_leftMotor::set, m_rightMotor::set);`
Lambdas can be passed to interoperate with vendor motor controller APIs that don't have e.g., set(double), so CTRE doesn't have to maintain their WPI_ classes anymore.
MotorControllerGroup was replaced with PWMMotorController.addFollower() for PWM motor controllers. Users of CAN motor controllers should use their vendor's follower functionality.
