Don't allow to render child which is not in :lajter/children

[petterik]

Children of a component must be declared in :lajter/children and calls to (render-child this child) should throw exceptions when called with a child that's not in the already defined children.

{:lajter/children []
 :render (fn [this props state] 
         (lajter.core/render-child this Child))}
                                     ;; ^^^^^^ is not in :lajter/children
;; This should throw