Version 1.4.0

Changed

• Use more efficient new skip_varint() function when iterating over
  packed varints.
• Split decode_varint() function into two functions speeding up the
  common case where a varint is only one byte long.
• Introduce new class iterator_range used instead of std::pair of
  iterators. This way the objects can be used in range-based for loops.
  Read UPGRADING.md for details.
• Introduce new class data_view and functions using and returning it.
  Read UPGRADING.md for details.