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/lib/shared/cpp/fpga_clock.cpp

@@ -0,0 +1,21 @@
+/*----------------------------------------------------------------------------*/
+/* 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.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#include "HAL/cpp/fpga_clock.h"
+
+#include "HAL/HAL.h"
+
+namespace hal {
+constexpr fpga_clock::time_point fpga_clock::min_time;
+
+fpga_clock::time_point fpga_clock::now() noexcept {
+  int32_t status = 0;
+  uint64_t currentTime = HAL_GetFPGATime(&status);
+  if (status != 0) return epoch();
+  return time_point(std::chrono::microseconds(currentTime));
+}
+}  // namespace hal