Merge: model: introduce the bottom type

An old branch that was on some disk that I forgot to PR.

While the bottom type is useless now, it should be useful on future PR.
Type theorists and lattice fanboys might like it.

Pull-Request: #1374
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>