[feature] Implement only in

[rhwy]

FromElements.In(left).OnlyIn(right)

OnlyIn returns elements that are only present in rightand not in left

exemple:

var wrapped = new ListElementsWrapper<int> (new []{ 1, 2, 3 });
var joinList = wrapped.OnlyIn (new List<int>{ 1, 3, 4 });

Check.That (joinList).ContainsExactly (4);