mruby-enumerator: add Enumerator#size

If the original object responds to `size` method, `Enumerator#size` calls
that method, otherwise return `nil`. CRuby returns the exact size for
more cases (e.g. Enumerable::Lazy#size), but mruby has more limitations.

mrbgems/mruby-enumerator/mrblib/enumerator.rb

@@ -238,6 +238,14 @@ class Enumerator
     end
   end
 
+  def size
+    if @size
+      @size
+    elsif @obj.respond_to?(:size)
+      @obj.size
+    end
+  end
+
   ##
   # call-seq:
   #   enum.each { |elm| block }                    -> obj