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.
Easier then the last one that put everything in a sub namespace. By
prefixing the name less things break, and intellisense will be less
confusing to new users during the transition.
