Diff simplifications

[jfmengels]

Dict.diff (Dict.fromList [ a, b ]) (Dict.fromList [ a, c ])
-->
Dict.diff (Dict.fromList [ b ]) (Dict.fromList [ c ])

Dict.diff (Dict.fromList [ ( a, b ), ( c, d ) ]) (Dict.fromList [ ( a, b ), ( e, f ) ])
-->
Dict.diff (Dict.fromList [ ( c, d ) ]) (Dict.fromList [ ( e, f ) ])

Dict.diff (Dict.fromList [ ( a, b ), ( c, d ) ]) (Dict.singleton a e)
--> Dict.fromList [ ( c, d ) ]

Dict.diff (Dict.singleton a e) (Dict.fromList [ ( a, b ), ( c, d ) ])
--> Dict.empty

Set.diff (Set.fromList [ a, b ]) (Set.fromList [ a, c ])
-->
Set.diff (Set.fromList [ b ]) (Set.fromList [ c ])

Set.diff (Set.fromList [ a, b ]) (Set.singleton [ a ])
-->
Set.empty

Set.diff (Set.singleton [ a ]) (Set.fromList [ a, b ])
-->
Set.empty

This shouldn't apply if expectNaN is enabled, unless the keys are literals.