wpiutil: Add C++ libuv wrappers (#1166)

- Provide an EventLoopRunner to run uv::Loop on a separate thread.

- Add raw_ostream wrapper for uv::Buffer.

wpiutil/src/main/native/include/wpi/EventLoopRunner.h

@@ -0,0 +1,50 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2018 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.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#ifndef WPIUTIL_WPI_EVENTLOOPRUNNER_H_
+#define WPIUTIL_WPI_EVENTLOOPRUNNER_H_
+
+#include <functional>
+
+#include "wpi/SafeThread.h"
+#include "wpi/uv/Loop.h"
+
+namespace wpi {
+
+/**
+ * Executes an event loop on a separate thread.
+ */
+class EventLoopRunner {
+ public:
+  EventLoopRunner();
+  virtual ~EventLoopRunner();
+
+  /**
+   * Run a function asynchronously (once) on the loop.
+   * This is safe to call from any thread, but is NOT safe to call from the
+   * provided function (it will deadlock).
+   * @param func function to execute on the loop
+   */
+  void ExecAsync(std::function<void(wpi::uv::Loop&)> func);
+
+  /**
+   * Run a function synchronously (once) on the loop.
+   * This is safe to call from any thread, but is NOT safe to call from the
+   * provided function (it will deadlock).
+   * This does not return until the function finishes executing.
+   * @param func function to execute on the loop
+   */
+  void ExecSync(std::function<void(wpi::uv::Loop&)> func);
+
+ private:
+  class Thread;
+  wpi::SafeThreadOwner<Thread> m_owner;
+};
+
+}  // namespace wpi
+
+#endif  // WPIUTIL_WPI_EVENTLOOPRUNNER_H_