Stronger "generic proper" lemmas for `big_op{L,M,S,MS}`.

- Renamed them from `_forall` into `_gen_proper`, to avoid confusion
  with `big_sep{L,M,S,MS}_forall`, which are actually about `∀`.
- For lists and maps there now two variants, `_gen_proper_2`, in case
  the maps or lists on both sides are different, and `_gen_proper`,
  in case the maps or lists on both sides are the same.