Write parser log file

Fix bug of skipping rule set IDs.
Remove unneeded out_sets from ItemSet class.

lib/propane/parser/item_set.rb

@@ -21,10 +21,6 @@ class Propane
       #   ItemSets leading to this item set.
       attr_reader :in_sets
 
-      # @return [Hash]
-      #   ItemSets reached from this item set. Key is a Token or Rule.
-      attr_reader :out_sets
-
       # @return [nil, Rule, Hash]
       #   Reduce actions, mapping lookahead tokens to rules.
       attr_accessor :reduce_actions
@@ -37,7 +33,6 @@ class Propane
         @items = Set.new(items)
         @following_item_set = {}
         @in_sets = Set.new
-        @out_sets = {}
         close!
       end