allow swapping the default registry without tears

[ikitommi]

(require '[malli.core :as m]
         '[malli.util :as mu]
         '[malli.registry :as mr]
         '[malli.generator :as mg])

;; look ma, just works
(mr/set-default-registry!
  (mr/composite-registry
    (m/default-schemas)
    (mu/schemas)))

(mg/generate
  [:merge
   [:map [:x :int]]
   [:map [:y :int]]])
; => {:x 0, :y 92}

[ikitommi]

related discussion: #565