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/uv/Async.h

@@ -0,0 +1,68 @@
+/*----------------------------------------------------------------------------*/
+/* 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_UV_ASYNC_H_
+#define WPIUTIL_WPI_UV_ASYNC_H_
+
+#include <uv.h>
+
+#include <memory>
+
+#include "wpi/Signal.h"
+#include "wpi/uv/Handle.h"
+
+namespace wpi {
+namespace uv {
+
+class Loop;
+
+/**
+ * Async handle.
+ * Async handles allow the user to "wakeup" the event loop and have a signal
+ * generated from another thread.
+ */
+class Async final : public HandleImpl<Async, uv_async_t> {
+  struct private_init {};
+
+ public:
+  explicit Async(const private_init&) {}
+  ~Async() noexcept override = default;
+
+  /**
+   * Create an async handle.
+   *
+   * @param loop Loop object where this handle runs.
+   */
+  static std::shared_ptr<Async> Create(Loop& loop);
+
+  /**
+   * Create an async handle.
+   *
+   * @param loop Loop object where this handle runs.
+   */
+  static std::shared_ptr<Async> Create(const std::shared_ptr<Loop>& loop) {
+    return Create(*loop);
+  }
+
+  /**
+   * Wakeup the event loop and emit the event.
+   *
+   * It’s safe to call this function from any thread.
+   * An async event will be emitted on the loop thread.
+   */
+  void Send() { Invoke(&uv_async_send, GetRaw()); }
+
+  /**
+   * Signal generated (on event loop thread) when the async event occurs.
+   */
+  sig::Signal<> wakeup;
+};
+
+}  // namespace uv
+}  // namespace wpi
+
+#endif  // WPIUTIL_WPI_UV_ASYNC_H_