Add `set_map_2` and associated lemmas.

Closed Dan Frumin requested to merge dfrumin/coq-stdpp:set_map_2 into master

The function set_map_2 generalizes the mapping function set_map, in that it ranges over two sets, and applies a given function to all the possible combinations of the elements.

I tried to recreate the same lemmas for set_map_2 as the ones that we already have for set_map.

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