[v0.1.1] -> Add retrieval methods for Predecessor & Successor

[sgoudham]

Context

Common operations surrounding BST's are retrieving Predecessor & Successor Successors.
The definitions of both terms are as follows:

Successor

The Successor can be thought of as the smallest value greater than the value of the given node.

E.G. Given a tree that looks like:

graph TB;
    A((8))-->B((3))
    A-->C((10))
    B-->D((1))
    B-->E((6))
    C-->F((9))
    C-->G((14))
    E-->H((4))
    E-->I((7))

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The Successor of 3 is 4.

Predecessor

The Predecessor can be thought of as the greatest value less than the value of the given node.

E.G Given a tree that looks like:

graph TB;
    A((8))-->B((3))
    A-->C((10))
    B-->D((1))
    B-->E((6))
    C-->F((9))
    C-->G((14))
    E-->H((4))
    E-->I((7))

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The Predecessor of 8 is 7.

What We Need To Do

Successor & Predecessor retrieval methods should implemented as part of the BinarySearchTree trait and implemented within RecursiveBST & IterativeBST