do-a-thing now does a thing and uses let.
Implemented spiff and cutify.
Implemented spiff-destructuring.
Implemented width and height.
Implemented square?.
Implemented area.
Implemented contains-point?.
Implemented contains-rectangle?.
Implemented title-length.
Implemented author-count.
Implemented multiple-authors?.
Implemented add-author.
Implemented alive?.
Implemented element-lengths and second-elements.
Implemented titles and stars.
Implemented monotonic?.
Implemented toggle and contains-duplicates?.
Implemented old-book->new-book.
Implemented has-author? authors and all-author-names.
Implemented author->string and authors->string.