18: Add solution for "quiz3".

Dependencies

• [2] H6XH45OP 0

Change contents

• edit in exercises/quiz3.rs at line 9
  [2.111312]→[2.111312:111330](∅→∅)
  −

  − // I AM NOT DONE
• replacement in exercises/quiz3.rs at line 20
  [2.111483]→[2.111483:111522](∅→∅)
  − assert_eq!(times_two(4), ???);
  [2.111483]

  [2.111522]
  + assert_eq!(times_two(4), 8);
• replacement in exercises/quiz3.rs at line 25
  [2.111586]→[2.111586:111687](∅→∅)
  − // TODO replace unimplemented!() with an assert for `times_two(-4)`

  − unimplemented!()
  [2.111586]

  [2.111687]
  + assert_eq!(times_two(-4), -8);