Add detection of unused struct fields

This uncovered some dead code, most notably in middle/liveness.rs, which I think suggests there must be something fishy with that part of the code.

The #[allow(dead_code)] annotations on some of the fields I am not super happy about but as I understand, marker type may disappear at some point.
I think this field is here on purpose, to stop it being able to be constructed outside this module (since the field is private).