Moves all CommandBase functionality into Command and deprecates CommandBase for removal.
Moves all SubsystemBase functionality into Subsystem and deprecates SubsystemBase for removal.
Adds a function to CommandScheduler to remove all registered Subsystems.
A lot of these are breaking changes. frc::Timer was replaced with the
contents of frc2::Timer. The others were in-place argument changes or
removing deprecated non-unit overloads.