Add splay tree data structure

[kuchma19]

Adds a SplayTree data structure.

Description

A splay tree is a self-adjusting binary search tree: after an access, the accessed value is moved to the root. If the searched value is absent, the last visited node is moved to the root instead. This implementation uses safe Rust with Option<Box<Node<T>>> and does not store parent pointers.

Implemented operations:

• new
• len
• is_empty
• contains
• insert
• remove
• merge
• FromIterator

The merge operation assumes that every value in the left tree is less than every value in the right tree.

Reference:
https://en.wikipedia.org/wiki/Splay_tree

Type of change

• New feature (non-breaking change which adds functionality)

Checklist:

• I ran bellow commands using the latest version of rust nightly.
• I ran cargo clippy --all -- -D warnings just before my last commit and fixed any issue that was found.
• I ran cargo fmt just before my last commit.
• I ran cargo test just before my last commit and all tests passed.
• I added my algorithm to the corresponding mod.rs file within its own folder, and in any parent folder(s).
• I added my algorithm to DIRECTORY.md with the correct link.
• I checked COUNTRIBUTING.md and my code follows its guidelines.

Notes:

• cargo fmt -- --check passes.
• cargo test splay_tree --lib passes.
• cargo clippy --all -- -D warnings currently reports existing warnings in unrelated files:
  • src/string/manacher.rs
  • src/greedy/job_sequencing.rs
  • src/string/burrows_wheeler_transform.rs
• Full cargo test aborted with SIGABRT while running the repository-wide test suite, outside the new splay_tree tests.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 95.90164% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.82%. Comparing base (37c9e12) to head (f4dfb53).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/data_structures/splay_tree.rs	95.90%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1041      +/-   ##
==========================================
- Coverage   95.82%   95.82%   -0.01%     
==========================================
  Files         393      394       +1     
  Lines       29972    30215     +243     
==========================================
+ Hits        28722    28954     +232     
- Misses       1250     1261      +11     

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