intruducing null type as list's end
common value class introducing

A value class could be accessed by a `revo` program with being interpreted as
a data structure that could be used in revo program.

A value class must implement the following method, most of them are
already implemented by default:

    * initialize(*)
      contructor
    * atom?
      whether this object is an atom or a list?
    * list?
      *NO* need to rewrite this function, since it will take the
      opposite value from `atom?`
    * is_true?
      whether is object repesent true or false in a conditional expression
    * inspect
      used in debugging printing
    * to_s
      used for commonly outputing
    * eval(env)
      return itself for literals, and the evaled result for
      evaluatable values
done some TODOs for `-`, `*` and `/` operators