Write a function to add an element to a binary search tree.

scala> End.addValue(2)
res0: Node[Int] = T(2 . .)

scala> res0.addValue(3)
res1: Node[Int] = T(2 . T(3 . .))

scala> res1.addValue(0)
res2: Node[Int] = T(2 T(0 . .) T(3 . .))