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

[nyurik]

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

[casey]

This seems like a pretty reasonable request.