Adds chrono wrapper around fpga time (#527)

Useful for some of the scenarios in C++ where using the chrono wrappers
might be very helpful.

hal/include/HAL/cpp/fpga_clock.h

@@ -0,0 +1,32 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) FIRST 2017. 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 <chrono>
+#include <limits>
+
+namespace hal {
+
+class fpga_clock {
+ public:
+  typedef std::chrono::microseconds::rep rep;
+  typedef std::chrono::microseconds::period period;
+  typedef std::chrono::microseconds duration;
+  typedef std::chrono::time_point<fpga_clock> time_point;
+
+  static fpga_clock::time_point now() noexcept;
+  static constexpr bool is_steady = true;
+
+  static constexpr fpga_clock::time_point epoch() { return time_point(zero()); }
+
+  static constexpr fpga_clock::duration zero() { return duration(0); }
+
+  static constexpr time_point min_time{
+      time_point(duration(std::numeric_limits<duration::rep>::min()))};
+};
+}  // namespace hal