Use wpi::span instead of wpi::ArrayRef across all libraries (#3414)

- Remove ArrayRef.h
- Add SpanExtras.h for a couple of convenience functions

wpilibNewCommands/src/main/native/include/frc2/command/NotifierCommand.h

@@ -9,7 +9,7 @@
 
 #include <frc/Notifier.h>
 #include <units/time.h>
-#include <wpi/ArrayRef.h>
+#include <wpi/span.h>
 
 #include "frc2/command/CommandBase.h"
 #include "frc2/command/CommandHelper.h"
@@ -45,7 +45,7 @@ class NotifierCommand : public CommandHelper<CommandBase, NotifierCommand> {
    * @param requirements the subsystems required by this command
    */
   NotifierCommand(std::function<void()> toRun, units::second_t period,
-                  wpi::ArrayRef<Subsystem*> requirements = {});
+                  wpi::span<Subsystem* const> requirements = {});
 
   NotifierCommand(NotifierCommand&& other);