Changed

• Converted all exercises with assertions to test mode.

Fixed

• cow1: Reverted regression introduced by calling to_mut where it shouldn't have been called, and clarified comment.
• primitive_types3: Require at least an array of 100 elements.
• Removed hint comments when no hint exists for the exercise.
• as_ref_mut: Fixed a typo in a test function name.
• enums3: Fixed formatting with rustfmt.