code organize issue and test cases

[gengjiawen]

• Is it possible add code based on chapter
  I think this way is more progressive, friendly to compiler newbie.

This is the sample code from https://interpreterbook.com/, I really enjoyed this way.

• Add test cases

test case can make the code more playable, helpful understand the code and more fun.

[keleshev]

So far, the idea was to split the code into part-1 and part-2 folders; I'll play around to see if a more fine-grained approach will work…