Implements a “Size Binary Search Tree” (SBST). An SBST is like a regular Binary Search Tree, but it is also capable of finding out, given a number j, how many nodes in the tree hold numbers less than or equal to j, and it is capable of doing so in time proportional to the height of the tree. It does so by using an additional field in each node, called the “size” of the node, which keeps a count of the number of nodes in the left subtree of the current node, plus one for the current node.