Replace Token struct with integer type

Replace _TOKEN_COUNT with INVALID_TOKEN_ID.

lib/propane/grammar.rb

@@ -30,6 +30,10 @@ class Propane
       @ptypes["default"]
     end
 
+    def invalid_token_id
+      @tokens.size
+    end
+
     private
 
     def parse_grammar!

lib/propane/parser.rb

@@ -62,7 +62,7 @@ class Propane
         reduce_entries =
           case ra = item_set.reduce_actions
           when Rule
-            [{token_id: @grammar.tokens.size, rule_id: ra.id,
+            [{token_id: @grammar.invalid_token_id, rule_id: ra.id,
               rule_set_id: ra.rule_set.id, n_states: ra.components.size}]
           when Hash
             ra.map do |token, rule|