Ch14 - Add test for reducing Money of different currency

Chapter 14: Change, page 67
Ch14 - Update Money reduce to handle rates

Chapter 14: Change, page 67
Ch14 - Move rate logic to Bank

Chapter 14: Change, pages 67-68
Ch14 - Create Pair

Chapter 14: Change, page 69
Ch14 - Add rate storage to Bank

Chapter 14: Change, pages 69-70
Ch14 - Add test for identity rate

Chapter 14: Change, page 70
Ch14 - Update rate to handle identity

Chapter 14: Change, page 70