Exercise 13

Use assoc and conj to write the function (add-author book new-author) that takes a book and an author as a parameter and adds author to books authors.

Hint: use let to avoid pain

(add-author little-schemer {:name "Gerald J. Sussman"})
;=> {:title "The Little Schemer"
;    :authors [{:birth-year 1944, :name "Daniel Friedman"}
;              {:name "Matthias Felleisen"}
;              {:name "Gerald J. Sussman"}]}
(add-author {:authors [{:name "Juhana"}]} {:name "Jani"})
;=> {:authors [{:name "Juhana"} {:name "Jani"}]}