Add lemma `map_subseteq_inv`.

Merged Robbert Krebbers requested to merge robbert/map_subseteq_inv into master

As pointed out by @swils in 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.

Edited by Robbert Krebbers

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