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/Prepare.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/Prepare.h"
+
+#include "wpi/uv/Loop.h"
+
+namespace wpi {
+namespace uv {
+
+std::shared_ptr<Prepare> Prepare::Create(Loop& loop) {
+  auto h = std::make_shared<Prepare>(private_init{});
+  int err = uv_prepare_init(loop.GetRaw(), h->GetRaw());
+  if (err < 0) {
+    loop.ReportError(err);
+    return nullptr;
+  }
+  h->Keep();
+  return h;
+}
+
+void Prepare::Start() {
+  Invoke(&uv_prepare_start, GetRaw(), [](uv_prepare_t* handle) {
+    Prepare& h = *static_cast<Prepare*>(handle->data);
+    h.prepare();
+  });
+}
+
+}  // namespace uv
+}  // namespace wpi