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

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

wpiutil/src/main/native/include/wpi/SpanExtras.h

@@ -0,0 +1,27 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#pragma once
+
+#include <cassert>
+
+#include "wpi/span.h"
+
+namespace wpi {
+
+/// Drop the first \p N elements of the array.
+template <typename T>
+constexpr span<T> drop_front(span<T> in, typename span<T>::size_type n = 1) {
+  assert(in.size() >= n && "Dropping more elements than exist");
+  return in.subspan(n, in.size() - n);
+}
+
+/// Drop the last \p N elements of the array.
+template <typename T>
+constexpr span<T> drop_back(span<T> in, typename span<T>::size_type n = 1) {
+  assert(in.size() >= n && "Dropping more elements than exist");
+  return in.subspan(0, in.size() - n);
+}
+
+}  // namespace wpi