checker: Adapt to changes in lib

fja_checker/__main__.py

 import lib.reg as reg
 import lib.cfl as cfl
-from lib.parsing import Parser, ParsingError
+from lib import parser
+from lib.parser import ParsingError
 from lib.checker import get_task, dfa_transform, nfa_transform, check_task, check_empty, check_alphabets, exit_correct, exit_incorrect
 import sys
 import signal
@@ -95,19 +96,18 @@ def exit_cfl_ok_but_invalid_constraint(msg : str) -> None:
 
 def cfg_task(teacher_type: str, teacher_string: str, task: str,
              student_string: str) -> None:
-    parser = Parser()
     assert task[:3] == "CFG", f"Invalid task prefix {task[:3]}"
     if len(task) > 3:
         assert task[3] == '+', f"Invalid task suffix {task[3:]}"
     constraints = filter(lambda x: len(x) != 0, task[4:].split(","))
     try:
-        teacher_solution = parser.str_to_cfg(teacher_string)
+        teacher_solution = parser.cfg(teacher_string)
     except ParsingError as message:
         print(paragraph(f"Error parsing teacher's solution: {message}"))
         exit(1)
 
     try:
-        student_solution = parser.str_to_cfg(student_string)
+        student_solution = parser.cfg(student_string)
     except ParsingError as message:
         print(paragraph(f"Error parsing student's solution: {message}"))
         exit(1)