- main topic
  + Object is parent of all class 
  + Implement skeleton of Rubyish::Hash, Rubyish::Array, Rubyish::String    # done
  + overload print function. If parameters is object in Rubyish, call #to_s automatically
  + C extesion, "" return String object, [] {} return Array and Hash object respectively.

- Object
  + #__id__         # done
  + #inspect 

- Class
  + #public
  + #private
  + #protected

- Kernel
  + #Hash()   #=> return Rubyish::Hash      # done
  + #Array()  #=> return Rubyish::Array     # done
  + #String() #=> return Rubyish::String    # done

- Module

- Hash

- Array

- String

