:thing/children abstract attribute

[acobster]

No more :post/children, :taxon/children, etc. It shouldn't matter what "type" it is. What we're interested in is:

1. whether the thing has children
2. whether the thing has a parent
3. whether we need to worry about ancestry when building a URL

If a thing has a :thing/_children attribute, we know it is the child of something. In combination with a slug abstraction, this can help us build URIs from route definitions in an abstract way, without caring about whether the thing is a post, taxon, menu item, etc.

(def route
  ["/{field/lang}/page/{entity/slug*}"
   {:name :page
    :route/parsed [:field/lang "page" {:attr :thing/slug
                                       :spec '{:thing/_children [:thing/slug {:thing/_children ...}}]}]}])