[project 11] refactor to make code more maintainable

[mtx2d]

1. remove global counters
   • self.if_level and self.while_level is error prone because it is increased at one place and reset from a different place.
2. remove not used source file
   • vm_writer.py is not used. Either refactor and use the file or remove it
3. break the giant test_compilation_engine.py into smaller tests
   • the test file is currently 4000 lines, break it into smaller tests.