Set.split and Set.splitMember should be strict in the key

[meooow25]

λ> Map.split undefined Map.empty
*** Exception: Prelude.undefined
λ> Map.splitLookup undefined Map.empty
*** Exception: Prelude.undefined
λ> Set.split undefined Set.empty
(fromList [],fromList [])
λ> Set.splitMember undefined Set.empty
(fromList [],False,fromList [])

This is similar to #982 and #983, unfortunately I didn't check Set at the time.

This changes behavior, so it will have to wait for the next major release.