Add `printRelativeTo`

[natefaubion]

Sometimes you have a Sandboxed path, but it needs to be printed relative to some other directory, which may involve ../. relativeTo does not handle ParentIn for security reasons, so it's not possible to print a path like this safely. Adding a printRelativeTo function would allow one to take an absolute sandboxed path, and print it relative to another arbitrary absolute path.

[garyb]

I think we can close this now as it's probably not as relevant as it was - sandboxing, relativeTo and printing of paths are all different now 🙂