Figure out what to do with relDirToPosix' and relFileToPosix'

These are unsafe versions of relDirToPosix and relFileToPosix.

I didn't document them, I should probably do that.

But I am concerned about the name -> is it clear enough that they are unsafe? Should we name them relDirToPosixUnsafe? Should we even bother with them, maybe we should kick them out?