Unify :empty_list and :list domains in maps

[gldubc]

No description provided.

[josevalim]

Now we can include :list twice. I think we need to handle that a bit upstream. :S

[gldubc]

I believe this gets handled gracefully thanks to

  # TODO: Double check if we indeed want the union here
  # when we start using domain types from Elixir itself
  defp map_put_domain(domain, key, value) do
    Map.update(domain, key, if_set(value), &union(&1, value))
  end

So when we instantiate say, empty_list() to integer() and non_empty_list() to atom() from Elixir, this will be split into two pairs of bindings passed to the constructor closed_map or open_map, with twice the same key domain_key(:list). Then, the map will have :list bound to integer() or atom(). I just added a test for that. A typing test would be nice too perhaps.