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/raw_uv_ostream.h

@@ -0,0 +1,70 @@
+/*----------------------------------------------------------------------------*/
+/* 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_RAW_UV_OSTREAM_H_
+#define WPIUTIL_WPI_RAW_UV_OSTREAM_H_
+
+#include "wpi/ArrayRef.h"
+#include "wpi/SmallVector.h"
+#include "wpi/raw_ostream.h"
+#include "wpi/uv/Buffer.h"
+
+namespace wpi {
+
+/**
+ * raw_ostream style output to a SmallVector of uv::Buffer buffers.  Fixed-size
+ * buffers are allocated and appended as necessary to fit the data being output.
+ * The SmallVector need not be empty at start.
+ */
+class raw_uv_ostream : public raw_ostream {
+ public:
+  /**
+   * Construct a new raw_uv_ostream.
+   * @param bufs Buffers vector.  NOT cleared on construction.
+   * @param allocSize Size to allocate for each buffer; allocation will be
+   *                  performed using Buffer::Allocate().
+   */
+  raw_uv_ostream(SmallVectorImpl<uv::Buffer>& bufs, size_t allocSize)
+      : m_bufs(bufs),
+        m_alloc([=]() { return uv::Buffer::Allocate(allocSize); }) {
+    SetUnbuffered();
+  }
+
+  /**
+   * Construct a new raw_uv_ostream.
+   * @param bufs Buffers vector.  NOT cleared on construction.
+   * @param alloc Allocator.
+   */
+  raw_uv_ostream(SmallVectorImpl<uv::Buffer>& bufs,
+                 std::function<uv::Buffer()> alloc)
+      : m_bufs(bufs), m_alloc(alloc) {
+    SetUnbuffered();
+  }
+
+  ~raw_uv_ostream() override = default;
+
+  /**
+   * Returns an ArrayRef to the buffers.
+   */
+  ArrayRef<uv::Buffer> bufs() { return m_bufs; }
+
+  void flush() = delete;
+
+ private:
+  void write_impl(const char* data, size_t len) override;
+  uint64_t current_pos() const override;
+
+  SmallVectorImpl<uv::Buffer>& m_bufs;
+  std::function<uv::Buffer()> m_alloc;
+
+  // How much allocated space is left in the current buffer.
+  size_t m_left = 0;
+};
+
+}  // namespace wpi
+
+#endif  // WPIUTIL_WPI_RAW_UV_OSTREAM_H_