Big mess of cleanup for day 2

[MaybeJustJames]

Chapter 8: Dictionaries

• Flesh out and correct text a little
• Describe the correct way to loop over the keys in a dictionary
• Formatting

Chapter 9: Files

• Formatting
• Use context managers (with) everywhere
• Describe why with is better than manually calling open()/close()

Chapter 10: Functions

• Formatting
• Fleshed out and corrected text a little
• Cover an error students might encounter writing and using their own functions
• Restructure and flesh out section titled "Flexibility in functions"
  • Reference back to chapter 5 unpacking and *
  • Talk about keyword arguments
• Add a section with some advice on writing functions

Solutions

• Updated in line with the above changes

[tmuylder]

Sweet. Some minor changes related to dictionaries will follow in next commit & followed by typo fixes.