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/Loop.cpp

@@ -0,0 +1,64 @@
+/*----------------------------------------------------------------------------*/
+/* 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/Loop.h"
+
+using namespace wpi::uv;
+
+Loop::Loop(const private_init&) noexcept {
+#ifndef _WIN32
+  // Ignore SIGPIPE (see https://github.com/joyent/libuv/issues/1254)
+  static bool once = []() {
+    signal(SIGPIPE, SIG_IGN);
+    return true;
+  }();
+  (void)once;
+#endif
+}
+
+Loop::~Loop() noexcept {
+  if (m_loop) {
+    m_loop->data = nullptr;
+    Close();
+  }
+}
+
+std::shared_ptr<Loop> Loop::Create() {
+  auto loop = std::make_shared<Loop>(private_init{});
+  if (uv_loop_init(&loop->m_loopStruct) < 0) return nullptr;
+  loop->m_loop = &loop->m_loopStruct;
+  loop->m_loop->data = loop.get();
+  return loop;
+}
+
+std::shared_ptr<Loop> Loop::GetDefault() {
+  static std::shared_ptr<Loop> loop = std::make_shared<Loop>(private_init{});
+  loop->m_loop = uv_default_loop();
+  if (!loop->m_loop) return nullptr;
+  loop->m_loop->data = loop.get();
+  return loop;
+}
+
+void Loop::Close() {
+  int err = uv_loop_close(m_loop);
+  if (err < 0) ReportError(err);
+}
+
+void Loop::Walk(std::function<void(Handle&)> callback) {
+  uv_walk(m_loop,
+          [](uv_handle_t* handle, void* func) {
+            auto& h = *static_cast<Handle*>(handle->data);
+            auto& f = *static_cast<std::function<void(Handle&)>*>(func);
+            f(h);
+          },
+          &callback);
+}
+
+void Loop::Fork() {
+  int err = uv_loop_fork(m_loop);
+  if (err < 0) ReportError(err);
+}