* Remove the Debug exercise, and the time refactor exercise as they were
  already moved to sprint 2.
* Remove the vowel refactor exercise. It's not clear what its goals are,
  or what it's looking for. We can re-add something here if we think
  it's valuable.
* Make clear the implement exercises should use Jest - that's what the
  entire sprint was about.
* Make previously "stretch" exercises be not stretch, because we've
  removed several pieces.
* A bunch of copy edits where things were unclear or contained typos.