Clean up and double-check exercises

[shiffman]

Have to look at four places

• /code/LearningProcessing
• /code/LearningProcessing-p5.js
• https://github.com/shiffman/LearningProcessing
• https://github.com/shiffman/LearningProcessing-p5.js

  Chapters:

• [ ] chapter 1
• [ ] chapter 2
• [ ] chapter 3
• [ ] chapter 4
• [ ] chapter 5
• [ ] chapter 6
• [ ] chapter 7
• [ ] chapter 8
• [ ] chapter 9
• [ ] chapter 10
• [ ] chapter 11
• [ ] chapter 12
• [ ] chapter 13
• [ ] chapter 14
• [ ] chapter 15
• [ ] chapter 16
• [ ] chapter 17
• [ ] chapter 18
• [ ] chapter 19
• [ ] chapter 20
• [ ] chapter 21
• [ ] chapter 22
• [ ] chapter 23