Add support for comparing the 'eXtensible ARchive' (.XAR/.PKG) file format

CPython releases macOS .pkg files which follow the XAR format. I'm currently working on build reproducibility of CPython artifacts and noticed the diffoscope doesn't support this format yet so quickly put together support for the XAR format. Unfortunately I don't have any example .pkg or .xar files that are small enough so I'll need to manually create a few of those (without any help from Ubuntu, xar is basically a dead format outside of macOS!)