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/cpp/uv/NetworkStream.cpp

@@ -0,0 +1,34 @@
+/*----------------------------------------------------------------------------*/
+/* 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.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#include "wpi/uv/NetworkStream.h"
+
+namespace wpi {
+namespace uv {
+
+ConnectReq::ConnectReq() {
+  error = [this](Error err) { GetStream().error(err); };
+}
+
+void NetworkStream::Listen(int backlog) {
+  Invoke(&uv_listen, GetRawStream(), backlog,
+         [](uv_stream_t* handle, int status) {
+           auto& h = *static_cast<NetworkStream*>(handle->data);
+           if (status < 0)
+             h.ReportError(status);
+           else
+             h.connection();
+         });
+}
+
+void NetworkStream::Listen(std::function<void()> callback, int backlog) {
+  connection.connect(callback);
+  Listen(backlog);
+}
+
+}  // namespace uv
+}  // namespace wpi