Implement Access for Geef.Tree

Make Geef.Tree its own record so we can define the protocol
implementation for it. There shouldn't be any generic functions you'd
want to call on a tree record.

lib/geef/tree.ex

@@ -1,6 +1,6 @@
 defrecord Geef.TreeEntry, Record.extract(:geef_tree_entry, from: "src/geef_records.hrl")
 
-defmodule Geef.Tree do
+defrecord Geef.Tree, Record.extract(:geef_object, from: "src/geef_records.hrl") do
   alias Geef.Object
   alias Geef.TreeEntry
 
@@ -9,13 +9,13 @@ defmodule Geef.Tree do
   def lookup(repo, id) do
     case :geef_tree.lookup(repo, id) do
       {:ok, obj} ->
-        {:ok, Object.new obj}
+        {:ok, new obj}
       error ->
         error
     end
   end
 
-  def get(tree = Object[type: :tree], path) do
+  def get(tree, path) do
     case :geef_tree.get(rebind(tree), path) do
       {:ok, entry} ->
         {:ok, TreeEntry.new entry}
@@ -25,3 +25,14 @@ defmodule Geef.Tree do
   end
 
 end
+
+defimpl Access, for: Geef.Tree do
+
+  def access(tree, key) do
+    case Geef.Tree.get(tree, key) do
+      {:ok, entry} -> entry
+      {:error, _} -> nil
+    end
+  end
+
+end