Remove immediate_data, use immediate_data_exp instead.

Rename interface get_2nd_data/get_2nd_judge to get_right_item.
Add free function qme::travel_exp to travel compiled expression without recursion.
Refactor safe_execute/safe_delete -- use qme::travel_exp function.

Totally eliminate recursion in safe_execute/safe_delete (except question mark expression used as sub expression).

 actually, for data_exp, recursion is not fully eliminated neither.

Fix bug - with optimization level <= 2, the data order for divide operations should not be exchanged,
 for example, 'b / (b / 3)' should not be transformed to '3', since the latter cannot trigger divide zero for integer.

 please note that the same thing cannot be applied to judge_exp since we cannot impact the Short-Circuit Evaluation.

 this is because recursion in judge_exp is still not eliminated.
Add free function qme::safe_delete to free compiled expressions if
 your expression is too complicated to be freed from the destructor,
 you should also use O0 to compile the expression, it uses no recursion
 during the compilation too.

 your expression is too complicated to be executed using recursion mechanism,
 you should also use O0 to compile the expression, it uses no recursion
 during the compilation too.

Fully support O0.

 you'll temporarily use them when bugs been found with level 2 and 3 before I fix them.

Add depth and final optimization to judge_exp, just for the consistency.
Rename question_exp_parser to compiler.
Compiler supports compiling judgement expressions too.