Add lemma `map_subseteq_inv`.

As pointed out by @swils in https://mattermost.mpi-sws.org/iris/pl/wzsucg5swtrttqhsawdd8pzama this lemma is missing

It's all the way down the file since I need properties of ∖. If someone knows a (short) proof without, we can move the property up, closer towards other properties on the order.