Remove <filesystem> fallback

[goto-bus-stop]

genieutils already depends on <filesystem> so we don't lose anything by doing the same.

Includes a bunch of fixes to path conversion, where we were implicitly converting paths to strings, which is not portable, and I don't know why C++ even has that feature because it is a bad one.

The cmake changes here might also accidentally "fix" the miniz linking issues @twestura was running into—proper fix is in https://github.com/sandsmark/genieutils/pull/9 but now we're compiling genieutils and WK into the same static library and WK also includes the miniz code, so it all ends up linking fine (at least on CI).