Add `relative_path()` to help with multi-platform relpath computation

At first I was using relpath bash util in all the scripts, until I learnt that it totally breaks on MacOS which even forces bash to be ran as zsh.  It would be very helpful to have an OS-agnostic way to compute a relative path between two paths, i.e. similar to [`strip_prefix`](https://doc.rust-lang.org/std/path/struct.Path.html#method.strip_prefix)