Usage ./sim [-d] <filename>

- e.g. ./sim -d test.in

出现"> "提示符表示进入debug模式，支持如下指令： 

+ p <pc/symbol/label/funciton name>     
    - e.g. p pc, p c1, p t1
    - Print the value of the symbol
+ s <number>
    - e.g. s 10
    - Run n Step
+ u <number/function/label>
    - e.g. u 10, u f_g, u l1
    - Run until pc equal to number or until the function or label.
+ r
    - e.g. r
    - Disable the debug mode and run until the program exit.

Add the doc of TAC simulator debug mode.
Add the doc of TAC simulator debug mode.
