Add splay tree data structure

Author: kuchma19

DIRECTORY.md

@@ -113,6 +113,7 @@
     * [RB Tree](https://github.com/TheAlgorithms/Rust/blob/master/src/data_structures/rb_tree.rs)
     * [Segment Tree](https://github.com/TheAlgorithms/Rust/blob/master/src/data_structures/segment_tree.rs)
     * [Segment Tree Recursive](https://github.com/TheAlgorithms/Rust/blob/master/src/data_structures/segment_tree_recursive.rs)
+    * [Splay Tree](https://github.com/TheAlgorithms/Rust/blob/master/src/data_structures/splay_tree.rs)
     * [Stack Using Singly Linked List](https://github.com/TheAlgorithms/Rust/blob/master/src/data_structures/stack_using_singly_linked_list.rs)
     * [Treap](https://github.com/TheAlgorithms/Rust/blob/master/src/data_structures/treap.rs)
     * [Trie](https://github.com/TheAlgorithms/Rust/blob/master/src/data_structures/trie.rs)

src/data_structures/mod.rs

@@ -15,6 +15,7 @@ mod rb_tree;
 mod segment_tree;
 mod segment_tree_recursive;
 mod skip_list;
+mod splay_tree;
 mod stack_using_singly_linked_list;
 mod treap;
 mod trie;
@@ -40,6 +41,7 @@ pub use self::rb_tree::RBTree;
 pub use self::segment_tree::SegmentTree;
 pub use self::segment_tree_recursive::SegmentTree as SegmentTreeRecursive;
 pub use self::skip_list::SkipList;
+pub use self::splay_tree::SplayTree;
 pub use self::stack_using_singly_linked_list::Stack;
 pub use self::treap::Treap;
 pub use self::trie::Trie;