- Changes the agreed-upon design slightly to make types of functions
  clearer:
  - Previously `path.join` worked on a list of paths, but required all but
    the first component to relative. This is now split into two functions:
    - `path.append <path> <string>` takes care of appending a relative
      path to an absolute path.
    - `path.relative.join [ <string> ]` takes care of joining relative
      paths together
  - `path.normalise` -> `path.relative.normalise`, because we don't need
    normalisation on absolute paths, Nix already takes care of that, and
    we use the `path.relative` namespace for anything only relating to
    relative paths

- Some more bikeshedding for the `relativeTo` name. I think `relativeTo`
  is pretty good, but @fricklerhandwerk likes other suggestions more
- Adds some suggestions for partial ordering checks on paths
- Adds a `difference` function, which can take care of common prefix and
  subpath calculations between any number of paths.

Also lays down the assumptions we're making about paths, assumptions
which notably also make the library work with the lazy trees Nix PR
(without relying or interfering with any of its bugs)