[feature] Implement also in

# ForElements.In(left).AlsoIn(right)

It returns the list of elements that are `both` in `left` and in `right`. It should help to find existing elements that could be modified.

exemple:

	var wrapped = new ListElementsWrapper<int> (new []{ 1, 2, 3 });
	var joinList = wrapped.AlsoIn (new List<int>{ 1, 3, 4 });

	Check.That (joinList).ContainsExactly (1, 3);