Manipulate the Hierarchy of VCDs generated by ASI (OSCI) PoC SystemC Simulator

All signals are recorded in one hierarchy named 'SystemC' in VCD generated by SystemC simulator even though VCD can describe hierarchy.

Just modifying the header of VCD is sufficient to set hierarchy structure. The easiest way is to write such script in Ruby or Python. I once wrote such script but it was slow for a large VCD file.
It had to read and write the main part of VCD even the header is need to be modified.

I wrote a small program to manipulate VCD very quickly. Mapping a file to memory is key point to get a good performance. If the file size does not change, head of file can be modified and only the modified portion is written.

It is available at github.

It might have been easy if I used std::string, but I used string_view class to avoid copying strings for future program.