This is derived from `wp_forget_not_stuck` and a trivial preorder on stuckness bits. (The two lemmas are redundant, but I have examples where each seems more natural than the other.) I did *not* bake `wp_stuckness_mono` into `strong_mono` for two reasons. Mainly, I didn't see a nice way to combine the two proofs (beyond `cut`). Less important, changing the type of `wp_strong_mono` will break code.