Split the two command implementations into separate libraries (#2012)

This will allow us at the user code side to determine to include old commands, new commands or both.

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

@@ -0,0 +1,37 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2019 FIRST. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#pragma once
+
+#include <memory>
+#include <type_traits>
+#include <utility>
+
+#include "Command.h"
+
+namespace frc2 {
+
+/**
+ * CRTP implementation to allow polymorphic decorator functions in Command.
+ *
+ * <p>Note: ALWAYS create a subclass by extending CommandHelper<Base, Subclass>,
+ * or decorators will not function!
+ */
+template <typename Base, typename CRTP,
+          typename = std::enable_if_t<std::is_base_of_v<Command, Base>>>
+class CommandHelper : public Base {
+  using Base::Base;
+
+ public:
+  CommandHelper() = default;
+
+ protected:
+  std::unique_ptr<Command> TransferOwnership() && override {
+    return std::make_unique<CRTP>(std::move(*static_cast<CRTP*>(this)));
+  }
+};
+}  // namespace frc2