Ch13 - Add test that plus returns a Sum

Chapter 13: Make It, page 62
Ch13 - Get plus returns Sum test passing

Chapter 13: Make It, pages 62-63
Ch13 - Add test for reducing a Sum

Chapter 13: Make It, page 63
Ch13 - Update reduce to pass reduce Sum test

Chapter 13: Make It, page 63
Ch13 - Refactor reduce, move body from Bank to Sum

Chapter 13: Make It, pages 63-64
Ch13 - Add test for reducing Money

Chapter 13: Make It, page 64
Ch13 - Update reduce to pass reduce Money test

Chapter 13: Make It, page 64
Ch13 - Refactor reduce to be polymorphic

Chapter 13: Make It, pages 64-65