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/raw_uv_ostream.cpp

@@ -0,0 +1,40 @@
+/*----------------------------------------------------------------------------*/
+/* 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/raw_uv_ostream.h"
+
+#include <cstring>
+
+using namespace wpi;
+
+void raw_uv_ostream::write_impl(const char* data, size_t len) {
+  while (len > 0) {
+    // allocate additional buffers as required
+    if (m_left == 0) {
+      m_bufs.emplace_back(m_alloc());
+      // we want bufs() to always be valid, so set len=0 and keep track of the
+      // amount of space remaining separately
+      m_left = m_bufs.back().len;
+      m_bufs.back().len = 0;
+      assert(m_left != 0);
+    }
+
+    size_t amt = std::min(m_left, len);
+    auto& buf = m_bufs.back();
+    std::memcpy(buf.base + buf.len, data, amt);
+    data += amt;
+    len -= amt;
+    buf.len += amt;
+    m_left -= amt;
+  }
+}
+
+uint64_t raw_uv_ostream::current_pos() const {
+  uint64_t size = 0;
+  for (auto&& buf : m_bufs) size += buf.len;
+  return size;
+}