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/cpp/frc2/command/ProxyScheduleCommand.cpp

@@ -6,10 +6,15 @@
 
 using namespace frc2;
 
-ProxyScheduleCommand::ProxyScheduleCommand(wpi::ArrayRef<Command*> toSchedule) {
+ProxyScheduleCommand::ProxyScheduleCommand(
+    wpi::span<Command* const> toSchedule) {
   SetInsert(m_toSchedule, toSchedule);
 }
 
+ProxyScheduleCommand::ProxyScheduleCommand(Command* toSchedule) {
+  SetInsert(m_toSchedule, {&toSchedule, 1});
+}
+
 void ProxyScheduleCommand::Initialize() {
   for (auto* command : m_toSchedule) {
     command->Schedule();