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/Handle.cpp

@@ -0,0 +1,36 @@
+/*----------------------------------------------------------------------------*/
+/* 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/Handle.h"
+
+using namespace wpi::uv;
+
+Handle::~Handle() noexcept {
+  if (!m_closed) {
+    uv_close(m_uv_handle, [](uv_handle_t* uv_handle) { delete uv_handle; });
+  } else {
+    delete m_uv_handle;
+  }
+}
+
+void Handle::Close() noexcept {
+  if (!IsClosing()) {
+    uv_close(m_uv_handle, [](uv_handle_t* handle) {
+      Handle& h = *static_cast<Handle*>(handle->data);
+      h.closed();
+      h.Release();  // free ourselves
+    });
+    m_closed = true;
+  }
+}
+
+void Handle::AllocBuf(uv_handle_t* handle, size_t size, uv_buf_t* buf) {
+  auto& h = *static_cast<Handle*>(handle->data);
+  *buf = h.m_allocBuf(size);
+}
+
+void Handle::DefaultFreeBuf(Buffer& buf) { buf.Deallocate(); }