message: Constify BinaryIterator
diff: @@ -145,7 +145,7 @@ public:
  {
  }
  - BinaryIterator(BinaryColumn* col, size_t ndx)
  + BinaryIterator(const BinaryColumn* col, size_t ndx)
  : m_binary_col(col)
  , m_ndx(ndx)
  {
  @@ -169,7 +169,7 @@ public:
  private:
  bool end_of_data = false;
  - BinaryColumn* m_binary_col = nullptr;
  + const BinaryColumn* m_binary_col = nullptr;
  size_t m_ndx = 0;
  size_t m_pos = 0;
  BinaryData m_binary;
  