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/RunCommand.h

@@ -0,0 +1,41 @@
+/*----------------------------------------------------------------------------*/
+/* 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 "CommandBase.h"
+#include "CommandHelper.h"
+
+namespace frc2 {
+/**
+ * A command that runs a Runnable continuously.  Has no end condition as-is;
+ * either subclass it or use Command.WithTimeout() or
+ * Command.InterruptOn() to give it one.  If you only wish
+ * to execute a Runnable once, use InstantCommand.
+ */
+class RunCommand : public CommandHelper<CommandBase, RunCommand> {
+ public:
+  /**
+   * Creates a new RunCommand.  The Runnable will be run continuously until the
+   * command ends.  Does not run when disabled.
+   *
+   * @param toRun        the Runnable to run
+   * @param requirements the subsystems to require
+   */
+  RunCommand(std::function<void()> toRun,
+             std::initializer_list<Subsystem*> requirements);
+
+  RunCommand(RunCommand&& other) = default;
+
+  RunCommand(const RunCommand& other) = default;
+
+  void Execute();
+
+ protected:
+  std::function<void()> m_toRun;
+};
+}  // namespace frc2