Exception during adding an item may leave the item "partially" added

[akade]

If an item that has a non-unique key, it results in "partial adds".

The following test fails as 3 is added to the set even though it should not have been as it is not indexed.

IndexedSet<int> set = IndexedSetBuilder<int>.Create()
                                                                        .WithUniqueIndex(x => x / 2)
                                                                        .Build();

set.Add(2);
Assert.ThrowsException<ArgumentException>(() => set.Add(3));
Assert.IsFalse(set.Contains(3));