Holder of structured data consisting of named values. ValueHolder is immutable, so the set operations in this class
return another ValueHolder and never modify the original ValueHolder.
A(a, B(b, c))
{ a: apple, B:{b:book, c:car} }
val n1 = EmptyNode.set("a", apple) => Node(a -> Leaf(apple))
val n2 = n1.set("B.b", "book") => Node(a -> Leaf(apple), B -> EmptyNode.set("b", "book")) => Node(a -> apple, B -> Node(b -> Leaf(book)))
val n3 = n2.set("B.c", "car") => Node(a -> apple, B -> Node(b -> Leaf(book), c -> Leaf(car)))