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/SetUtilities.h

@@ -4,12 +4,12 @@
 
 #pragma once
 
-#include <wpi/ArrayRef.h>
 #include <wpi/SmallVector.h>
+#include <wpi/span.h>
 
 namespace frc2 {
 template <typename T>
-void SetInsert(wpi::SmallVectorImpl<T*>& vector, wpi::ArrayRef<T*> toAdd) {
+void SetInsert(wpi::SmallVectorImpl<T*>& vector, wpi::span<T* const> toAdd) {
   for (auto addCommand : toAdd) {
     bool exists = false;
     for (auto existingCommand : vector) {