Add new C++ Command framework (#1785)

This is the C++ version of #1682.

The old command framework is still available, but will be deprecated.

Due to name conflicts, the new framework is in the frc2 namespace.
Eventually (after the old command framework is removed in a future year)
it will be moved into the main frc namespace.

wpilibc/src/main/native/include/frc2/command/PerpetualCommand.h

@@ -0,0 +1,66 @@
+/*----------------------------------------------------------------------------*/
+/* 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 <utility>
+
+#include "CommandBase.h"
+#include "CommandGroupBase.h"
+#include "CommandHelper.h"
+
+namespace frc2 {
+/**
+ * A command that runs another command in perpetuity, ignoring that command's
+ * end conditions.  While this class does not extend {@link CommandGroupBase},
+ * it is still considered a CommandGroup, as it allows one to compose another
+ * command within it; the command instances that are passed to it cannot be
+ * added to any other groups, or scheduled individually.
+ *
+ * <p>As a rule, CommandGroups require the union of the requirements of their
+ * component commands.
+ */
+class PerpetualCommand : public CommandHelper<CommandBase, PerpetualCommand> {
+ public:
+  /**
+   * Creates a new PerpetualCommand.  Will run another command in perpetuity,
+   * ignoring that command's end conditions, unless this command itself is
+   * interrupted.
+   *
+   * @param command the command to run perpetually
+   */
+  explicit PerpetualCommand(std::unique_ptr<Command>&& command);
+
+  /**
+   * Creates a new PerpetualCommand.  Will run another command in perpetuity,
+   * ignoring that command's end conditions, unless this command itself is
+   * interrupted.
+   *
+   * @param command the command to run perpetually
+   */
+  template <class T, typename = std::enable_if_t<std::is_base_of<
+                         Command, std::remove_reference_t<T>>::value>>
+  explicit PerpetualCommand(T&& command)
+      : PerpetualCommand(std::make_unique<std::remove_reference_t<T>>(
+            std::forward<T>(command))) {}
+
+  PerpetualCommand(PerpetualCommand&& other) = default;
+
+  // No copy constructors for command groups
+  PerpetualCommand(const PerpetualCommand& other) = delete;
+
+  void Initialize() override;
+
+  void Execute() override;
+
+  void End(bool interrupted) override;
+
+ private:
+  std::unique_ptr<Command> m_command;
+};
+}  // namespace frc2