feat: classfile representation

[Chang-CH]

Thanks for adding the types and constants for ClassFile, the overall structure is very clean.

An alternative way to organize the files is as follows.

ClassFile/
  |_constants/
  |_types/

Since the AST part might also has its constants and types, I feel like organizing them this way gives better separation and fewer confusions since different parts can work on different top level folder.

Refactored.

[bryanlohxz]

So sorry, but can we place the ClassFile overarching folder under the src directory?

[1001mei]

Ya, sure!