Cache Node#html? and Node#root_node

The results of these methods is now cached until a Node is moved into
another NodeSet. This reduces the time spent in the
xpath/evaluator/big_xml_average_bench.rb benchmark from roughly 10
seconds to roughly 5 seconds per iteration.

lib/oga/xml/document.rb

@@ -12,7 +12,7 @@ module Oga
     #  The XML declaration of the document.
     #  @return [Oga::XML::XmlDeclaration]
     #
-    # @!attribute [rw] type
+    # @!attribute [r] type
     #  The document type, either `:xml` or `:html`.
     #  @return [Symbol]
     #
@@ -20,7 +20,9 @@ module Oga
       include Querying
       include Traversal
 
-      attr_accessor :doctype, :xml_declaration, :type
+      attr_accessor :doctype, :xml_declaration
+
+      attr_reader :type
 
       ##
       # @param [Hash] options

lib/oga/xml/node.rb

@@ -5,13 +5,13 @@ module Oga
     # {Oga::XML::NodeSet} and can be used to query surrounding and parent
     # nodes.
     #
-    # @!attribute [rw] node_set
+    # @!attribute [r] node_set
     #  @return [Oga::XML::NodeSet]
     #
     class Node
       include Traversal
 
-      attr_accessor :node_set
+      attr_reader :node_set
 
       ##
       # @param [Hash] options
@@ -23,11 +23,19 @@ module Oga
       #  the current node.
       #
       def initialize(options = {})
-        @node_set = options[:node_set]
-
+        self.node_set = options[:node_set]
         self.children = options[:children] if options[:children]
       end
 
+      ##
+      # @param [Oga::XML::NodeSet] set
+      #
+      def node_set=(set)
+        @node_set  = set
+        @root_node = nil
+        @html_p    = nil
+      end
+
       ##
       # Returns the child nodes of the current node.
       #
@@ -120,17 +128,21 @@ module Oga
       # @return [Oga::XML::Document|Oga::XML::Node]
       #
       def root_node
-        node = self
+        unless @root_node
+          node = self
 
-        loop do
-          if !node.is_a?(Document) and node.node_set
-            node = node.node_set.owner
-          else
-            break
+          loop do
+            if !node.is_a?(Document) and node.node_set
+              node = node.node_set.owner
+            else
+              break
+            end
           end
+
+          @root_node = node
         end
 
-        return node
+        return @root_node
       end
 
       ##
@@ -168,9 +180,13 @@ module Oga
       # @return [TrueClass|FalseClass]
       #
       def html?
-        root = root_node
+        if @html_p.nil?
+          root = root_node
 
-        return root.is_a?(Document) && root.html?
+          @html_p = root.is_a?(Document) && root.html?
+        end
+
+        return @html_p
       end
 
       ##