message: Allow move-construct, move-assign of AppendBuffer<T>
diff: @@ -121,6 +121,9 @@ public:
  {
  }
  + AppendBuffer(AppendBuffer&&) noexcept = default;
  + AppendBuffer& operator=(AppendBuffer&&) noexcept = default;
  +
  /// Returns the current size of the buffer.
  size_t size() const noexcept;
  