Add successor and predecessor methods for BSTs

[ybrenning]

I would like to add a method get_predecessor() and get_successor() to the BinarySearchTree class implementation.
These are common algorithms performed on Binary Search Trees and will return the next lowest/next highest value from the tree based on a given BST and a given value.

[LucasP78]

Hello, I am willing to work on this!

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[AdemolaAri]

Hi, I can work on this