Add a destructor for struct Circom_Circuit to release dynamically allocated memory and prevent memory leaks.

[yushihang]

close #23

[yushihang]

I would like to kindly bring to your attention that, while this approach does not fully adhere to the RAII (Resource Acquisition Is Initialization) principle, it does address the issue of memory leaks and is the minimal modification required to the code. :-)

Considering that the code for the three structs, IODef, IODefPair, and Circom_CalcWit, are organized/maintained together, future maintainers will find it convenient to make consistent modifications to the existing code.

Therefore, the current implementation approach was adopted.