Created a persistent binary search tree (PBST) and persistent stack. When adding a new element will create a new
stack or PBST with the new element and connect the new PBST or stack to the previous PBST or stack.
***PLEASE SEE "Persistent_BST_Add_element.png" *** for how adding an element to original PBST (x),
creates a new structure (y) and y structure connects to original structure x
