apply fn to all value of a map

*note: Clojure 1.11 now has new function: (update-vals m f) and (update-keys m f).

https://clojure.github.io/clojure/branch-master/clojure.core-api.html#clojure.core/update-vals

(into {} (for [[k v] m]
           [k (f v)]))

; using Specter
(transform [MAP-VALS] inc {:a 1 :b 2 :c 3})
;=> {:a 2, :b 3, :c 4}