[wpilib] PWMSpeedController: Use PWM by composition (#3248)

This cleans up the user experience by removing lower-level functions from the
interface.

Also remove MotorSafety from "raw" PWM.

wpilibc/src/main/native/include/frc/MotorSafety.h

@@ -5,11 +5,14 @@
 #pragma once
 
 #include <wpi/mutex.h>
-#include <wpi/raw_ostream.h>
 
 #include "frc/ErrorBase.h"
 #include "frc/Timer.h"
 
+namespace wpi {
+class raw_ostream;
+}  // namespace wpi
+
 namespace frc {
 
 /**