add prepend/append

; (def v {"a" 1, "b" 1})
(bisection-key.util/key-prepend v) ; "G"
(bisection-key.util/key-append v) ; "n"

(bisection-key.util/assoc-prepend v 2) ; (assoc v "G" 2)
(bisection-key.util/assoc-append v 2) ; (assoc v "n" 2)