Use rerunfilecheck to ensure that exercise information is up to date

[N-Coder]

When making structural changes to the exercise of a document (e.g. adding and removing or reordering), xsim sometimes doesn't pick up all the changes and keeps outdated information, leading to wrong information in the output file - even when using clear-aux (which according to docs only checks the number of exercises).
My recommendation would be to use the rerunfilecheck package to see if the MD5 hash of the document.xsim cache changed and only clear and rerun depending on this information.
The package only works with pdf- and luatex, but that should cover most of the uses.

[muzimuzhi]

XeTeX has added primitive \mdfivesum in 2016, and latex3 has provided engine-independent functions \file_mdfive_hash:n and \file_get_mdfive_hash:nNTF. So it's possible for xsim to do the MD5 check that works on all the common engines, by itself.

[cgnieder]

I implemented the md5 check for the next version.