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/FsEvent.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_FSEVENT_H_
+#define WPIUTIL_WPI_UV_FSEVENT_H_
+
+#include <uv.h>
+
+#include <memory>
+#include <string>
+
+#include "wpi/Signal.h"
+#include "wpi/Twine.h"
+#include "wpi/uv/Handle.h"
+
+namespace wpi {
+namespace uv {
+
+class Loop;
+
+/**
+ * Filesystem Event handle.
+ */
+class FsEvent final : public HandleImpl<FsEvent, uv_fs_event_t> {
+  struct private_init {};
+
+ public:
+  explicit FsEvent(const private_init&) {}
+  ~FsEvent() noexcept override = default;
+
+  /**
+   * Create a filesystem event handle.
+   *
+   * @param loop Loop object where this handle runs.
+   */
+  static std::shared_ptr<FsEvent> Create(Loop& loop);
+
+  /**
+   * Create a filesystem event handle.
+   *
+   * @param loop Loop object where this handle runs.
+   */
+  static std::shared_ptr<FsEvent> Create(const std::shared_ptr<Loop>& loop) {
+    return Create(*loop);
+  }
+
+  /**
+   * Start watching the specified path for changes.
+   *
+   * @param path Path to watch for changes
+   * @param events Bitmask of event flags.  Only UV_FS_EVENT_RECURSIVE is
+   *               supported (and only on OSX and Windows).
+   */
+  void Start(const Twine& path, unsigned int flags = 0);
+
+  /**
+   * Stop watching for changes.
+   */
+  void Stop() { Invoke(&uv_fs_event_stop, GetRaw()); }
+
+  /**
+   * Get the path being monitored.  Signals error and returns empty string if
+   * an error occurs.
+   * @return Monitored path.
+   */
+  std::string GetPath();
+
+  /**
+   * Signal generated when a filesystem change occurs.  The first parameter
+   * is the filename (if a directory was passed to Start(), the filename is
+   * relative to that directory).  The second parameter is an ORed mask of
+   * UV_RENAME and UV_CHANGE.
+   */
+  sig::Signal<const char*, int> fsEvent;
+};
+
+}  // namespace uv
+}  // namespace wpi
+
+#endif  // WPIUTIL_WPI_UV_FSEVENT_H_