Makes slots linked list

slots used to ba an array of symbols
Now we have an object for each slot, that holds the name and the next_slot
relatively easy change, though quite broad

lib/sol/return_statement.rb

@@ -32,7 +32,7 @@ module Sol
 
     def slot_load(compiler)
       SlotMachine::SlotLoad.new( self , [:message , :return_value] ,
-                        @return_value.to_slot_definition(compiler) )
+                        @return_value.to_slotted(compiler) )
     end
   end
 end