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/Idle.h

@@ -0,0 +1,79 @@
+/*----------------------------------------------------------------------------*/
+/* 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_IDLE_H_
+#define WPIUTIL_WPI_UV_IDLE_H_
+
+#include <uv.h>
+
+#include <memory>
+
+#include "wpi/Signal.h"
+#include "wpi/uv/Handle.h"
+
+namespace wpi {
+namespace uv {
+
+class Loop;
+
+/**
+ * Idle handle.
+ *
+ * Idle handles will generate a signal once per loop iteration, right
+ * before the Prepare handles.
+ *
+ * The notable difference with Prepare handles is that when there are active
+ * idle handles, the loop will perform a zero timeout poll instead of blocking
+ * for I/O.
+ *
+ * @warning Despite the name, idle handles will signal every loop iteration,
+ * not when the loop is actually "idle".  This also means they can easly become
+ * CPU hogs.
+ */
+class Idle final : public HandleImpl<Idle, uv_idle_t> {
+  struct private_init {};
+
+ public:
+  explicit Idle(const private_init&) {}
+  ~Idle() noexcept override = default;
+
+  /**
+   * Create an idle handle.
+   *
+   * @param loop Loop object where this handle runs.
+   */
+  static std::shared_ptr<Idle> Create(Loop& loop);
+
+  /**
+   * Create an idle handle.
+   *
+   * @param loop Loop object where this handle runs.
+   */
+  static std::shared_ptr<Idle> Create(const std::shared_ptr<Loop>& loop) {
+    return Create(*loop);
+  }
+
+  /**
+   * Start the handle.
+   */
+  void Start();
+
+  /**
+   * Stop the handle.  The signal will no longer be generated.
+   */
+  void Stop() { Invoke(&uv_idle_stop, GetRaw()); }
+
+  /**
+   * Signal generated once per loop iteration prior to Prepare signals.
+   */
+  sig::Signal<> idle;
+};
+
+}  // namespace uv
+}  // namespace wpi
+
+#endif  // WPIUTIL_WPI_UV_IDLE_H_