Adding a ast_util.py to visualize ASTs.

[baiqiushi]

Introducing an ast_util.py to visualize the AST tree and help reveiw test cases.

The visualization result is like the following:

tests/test_query_parser.py::test_subquery_parse 
-------------------------------- live log call --------------------------------
INFO     tests.test_query_parser:test_query_parser.py:145 
============================================================================
SQL QUERY                             AST STRUCTURE
============================================================================
                                      +- query
SELECT empno,                         | +- select
       firstnme,                      | | +- column: empno
       lastname,                      | | +- column: firstnme
       phoneno                        | | +- column: lastname
FROM employee                         | | +- column: phoneno
WHERE workdept IN                     | +- from
    (SELECT deptno                    | | +- table: employee
     FROM department                  | +- where
     WHERE deptname = 'OPERATIONS')   | | +- operator: AND
  AND 1=1;                            | | | +- operator: IN
                                      | | | | +- column: workdept
                                      | | | | +- subquery
                                      | | | | | +- query
                                      | | | | | | +- select
                                      | | | | | | | +- column: deptno
                                      | | | | | | +- from
                                      | | | | | | | +- table: department
                                      | | | | | | +- where
                                      | | | | | | | +- operator: =
                                      | | | | | | | | +- column: deptname
                                      | | | | | | | | +- literal: OPERATIONS
                                      | | | +- operator: =
                                      | | | | +- literal: 1
                                      | | | | +- literal: 1
============================================================================

[HazelYuAhiru]

LGTM, thanks a lot!

The base branch was changed.