Fixing usability of nested lattice types in the #[derive(Lattice)] macro

[MingweiSamuel]

Rohit ran into this - want to use the #[derive(Lattice)] macro, but then you can't use the inner fields for a LatticeBimorphism

Possible fixes:

1. Allow mapping of state references - something we have thought about for a long time. If we could do something like state() -> map(|x| &x.system) to map the state reference (not the items) and use that reference in the lattice bimorphism, that work work.
2. Allow some sort of #[derive(LatticeBimorphism)] macro in the same was as deriving Lattice. But kinda clunky, have to align fields.

[rohitkulshreshtha]

Next steps: Design needed. Move to a feature branch until we have a design ready.