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/test/native/cpp/raw_uv_stream_test.cpp

@@ -0,0 +1,58 @@
+/*----------------------------------------------------------------------------*/
+/* 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"  // NOLINT(build/include_order)
+
+#include "gtest/gtest.h"
+
+namespace wpi {
+
+TEST(RawUvStreamTest, BasicWrite) {
+  SmallVector<uv::Buffer, 4> bufs;
+  raw_uv_ostream os(bufs, 1024);
+  os << "12";
+  os << "34";
+  ASSERT_EQ(bufs.size(), 1u);
+  ASSERT_EQ(bufs[0].len, 4u);
+  ASSERT_EQ(bufs[0].base[0], '1');
+  ASSERT_EQ(bufs[0].base[1], '2');
+  ASSERT_EQ(bufs[0].base[2], '3');
+  ASSERT_EQ(bufs[0].base[3], '4');
+}
+
+TEST(RawUvStreamTest, BoundaryWrite) {
+  SmallVector<uv::Buffer, 4> bufs;
+  raw_uv_ostream os(bufs, 4);
+  ASSERT_EQ(bufs.size(), 0u);
+  os << "12";
+  ASSERT_EQ(bufs.size(), 1u);
+  os << "34";
+  ASSERT_EQ(bufs.size(), 1u);
+  os << "56";
+  ASSERT_EQ(bufs.size(), 2u);
+}
+
+TEST(RawUvStreamTest, LargeWrite) {
+  SmallVector<uv::Buffer, 4> bufs;
+  raw_uv_ostream os(bufs, 4);
+  os << "123456";
+  ASSERT_EQ(bufs.size(), 2u);
+  ASSERT_EQ(bufs[1].len, 2u);
+  ASSERT_EQ(bufs[1].base[0], '5');
+}
+
+TEST(RawUvStreamTest, PrevDataWrite) {
+  SmallVector<uv::Buffer, 4> bufs;
+  bufs.emplace_back(uv::Buffer::Allocate(1024));
+  raw_uv_ostream os(bufs, 1024);
+  os << "1234";
+  ASSERT_EQ(bufs.size(), 2u);
+  ASSERT_EQ(bufs[0].len, 1024u);
+  ASSERT_EQ(bufs[1].len, 4u);
+}
+
+}  // namespace wpi