Restore option_map in FMapFacts
Restore option_map in FMapFacts

This definition was removed by a4043608f704f026de7eb5167a109ca48e00c221 (This commit adds full universe polymorphism and fast projections to Coq), for reasons I do not know.  This means that things like `unfold option_map` work only in 8.5, while `unfold <application of Facts>.option_map` works only in 8.4.  This allows `unfold` to work correctly in both 8.4 and 8.5.
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