Covariant persistent multiset

An immutable multiset can expose a covariant type parameter because reading an element only produces that type. Addition accepts a new type parameter with a lower type bound, add[U >: T](x: U), and returns a new ISet[U]. The old ISet[T] retains its contents and original element guarantee. Deletion returns both the removed value and a new remaining multiset; it does not mutate the old one. Duplicate occurrences are preserved.