improve shrink/3 for lists

[mndrix]

It's important to shrink lists correctly and efficiently. Many other types rely upon good behavior here. For example, shrinking an atom uses this code (via list(codes)). However a property like

prop_atom_fail(A:atom) :-
    atom_length(A,Len),
    Len < 3.

gives different "minimal" results each time: bMe, 'a^4', a.3, etc. The length is fine (could be more efficient) but shrinking elements needs improvement for sure.