nix_expr: Merge lists in expand_dict() as well.

This is essentially duplicating some code from nixmerge(), but for now
it should fix collisions when merging something like these two attrsets:

a1 = {
  a.b = [ 1 2 ]; }
};

a2 = {
  a = {
    b = [ 3 4 ];
  }
};

Signed-off-by: aszlig <aszlig@redmoonstudios.org>