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/Signal.h

@@ -0,0 +1,84 @@
+/*----------------------------------------------------------------------------*/
+/* 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_SIGNAL_H_
+#define WPIUTIL_WPI_UV_SIGNAL_H_
+
+#include <uv.h>
+
+#include <memory>
+
+#include "wpi/Signal.h"
+#include "wpi/uv/Handle.h"
+
+namespace wpi {
+namespace uv {
+
+class Loop;
+
+/**
+ * Signal handle.
+ */
+class Signal final : public HandleImpl<Signal, uv_signal_t> {
+  struct private_init {};
+
+ public:
+  explicit Signal(const private_init&) {}
+  ~Signal() noexcept override = default;
+
+  /**
+   * Create a signal handle.
+   *
+   * @param loop Loop object where this handle runs.
+   */
+  static std::shared_ptr<Signal> Create(Loop& loop);
+
+  /**
+   * Create a signal handle.
+   *
+   * @param loop Loop object where this handle runs.
+   */
+  static std::shared_ptr<Signal> Create(const std::shared_ptr<Loop>& loop) {
+    return Create(*loop);
+  }
+
+  /**
+   * Start watching for the given signal.
+   *
+   * @param signum Signal to watch for.
+   */
+  void Start(int signum);
+
+  /**
+   * Start watching for the given signal.  Same as Start() but the signal
+   * handler is reset the moment the signal is received.
+   *
+   * @param signum Signal to watch for.
+   */
+  void StartOneshot(int signum);
+
+  /**
+   * Stop watching for the signal.
+   */
+  void Stop() { Invoke(&uv_signal_stop, GetRaw()); }
+
+  /**
+   * Get the signal being monitored.
+   * @return Signal number.
+   */
+  int GetSignal() const { return GetRaw()->signum; }
+
+  /**
+   * Signal generated when a signal occurs.
+   */
+  sig::Signal<int> signal;
+};
+
+}  // namespace uv
+}  // namespace wpi
+
+#endif  // WPIUTIL_WPI_UV_SIGNAL_H_