Remove MotorSafetyHelper, create MotorSafety base class instead (#562)

Most of the MotorSafety implementation was moved into the MotorSafety base
class. SafePWM's inheritance of MotorSafety was moved into PWM to
eliminate Java needing a helper class.

In Java, a helper class for Sendable (SendableImpl) was added due to
lack of multiple inheritance.

wpilibc/src/main/native/cpp/DriverStation.cpp

@@ -19,7 +19,7 @@
 #include <wpi/StringRef.h>
 
 #include "frc/AnalogInput.h"
-#include "frc/MotorSafetyHelper.h"
+#include "frc/MotorSafety.h"
 #include "frc/Timer.h"
 #include "frc/Utility.h"
 #include "frc/WPIErrors.h"
@@ -557,7 +557,7 @@ void DriverStation::Run() {
     if (IsDisabled()) safetyCounter = 0;
 
     if (++safetyCounter >= 4) {
-      MotorSafetyHelper::CheckMotors();
+      MotorSafety::CheckMotors();
       safetyCounter = 0;
     }
     if (m_userInDisabled) HAL_ObserveUserProgramDisabled();