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/Work.cpp

@@ -0,0 +1,48 @@
+/*----------------------------------------------------------------------------*/
+/* 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/Work.h"
+
+#include "wpi/uv/Loop.h"
+
+namespace wpi {
+namespace uv {
+
+WorkReq::WorkReq() {
+  error = [this](Error err) { GetLoop().error(err); };
+}
+
+void QueueWork(Loop& loop, const std::shared_ptr<WorkReq>& req) {
+  int err = uv_queue_work(loop.GetRaw(), req->GetRaw(),
+                          [](uv_work_t* req) {
+                            auto& h = *static_cast<WorkReq*>(req->data);
+                            h.work();
+                          },
+                          [](uv_work_t* req, int status) {
+                            auto& h = *static_cast<WorkReq*>(req->data);
+                            if (status < 0)
+                              h.ReportError(status);
+                            else
+                              h.afterWork();
+                            h.Release();  // this is always a one-shot
+                          });
+  if (err < 0)
+    loop.ReportError(err);
+  else
+    req->Keep();
+}
+
+void QueueWork(Loop& loop, std::function<void()> work,
+               std::function<void()> afterWork) {
+  auto req = std::make_shared<WorkReq>();
+  req->work.connect(work);
+  req->afterWork.connect(afterWork);
+  QueueWork(loop, req);
+}
+
+}  // namespace uv
+}  // namespace wpi