Implement m/merge

[ikitommi]

... one of the original reasons for malli was to be able deep-merge map-schemas easily. Not perfect (doesn't understand how to merge things like :and & :or), but a start. Code is kinda bloaty.

Will fix: #45

(m/merge
  [:map 
   [:parameters
    [:map
     [:query-params 
      [:map [:x int?]]]]]]
  [:map 
   [:parameters
    [:map
     [:query-params 
      [:map [:y int?]]]
     [:body-params
      [:map [:z int?]]]]]])
;[:map 
; [:parameters
;  [:map
;   [:query-params 
;    [:map [:x string?] [:y int?]]]
;   [:body-params
;    [:map [:z int?]]]]]]

[ikitommi]

Will go to the big internal review..