Add a "big map" backend

[arrdem]

This may force other API changes such as having a "connection" or some other state value bigger than the "configuration" currently used especially as "writing" to this datastore will just generate a new value whereas all the "writing" operations on the existing datastore(s) are for side-effects alone.

{"org.clojure"
 [:grimoire.things/group
  {:name     "org.clojure"
   :meta     {}
   :examples []
   :related  []
   :notes    ""
   :children
   {"clojure"
    [:grimoire.things/artifact
     {:name     "clojure"
      :meta     {}
      :examples []
      :related  []
      :notes    ""
      :children
      {"1.6.0"
       [:grimoire.things/version
        {:name     "1.6.o"
         :meta     {}
         :examples []
         :related  []
         :notes    ""
         :children
         {"clj"
          [:grimoire.things/platform
           {:name     "clj"
            :meta     {}
            :examples []
            :related  []
            :notes    ""
            :children
            {"clojure.core"
             [:grimoire.things/namespace
              {:name     "clojure.core"
               :meta     {}
               :examples []
               :related  []
               :notes    ""
               :children
               {"def"
                [:grimoire.things/def
                 {:name     "def"
                  :meta     {}
                  :examples []
                  :related  []
                  :notes    ""}]}}]}}]}}]}}]}}]}

One possible structure for this mapping. An open question is how notes and examples fit into this structure.

/cc @puredanger