add skip list

[zhangzhuang15]

Pull Request Template

Description

Add skip list into data structure.

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.

Please make sure that if there is a test that takes too long to run ( > 300ms), you #[ignore] that or
try to optimize your code or make the test easier to run. We have this rule because we have hundreds of
tests to run; If each one of them took 300ms, we would have to wait for a long time.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.75000% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.42%. Comparing base (7c1542b) to head (eecd3b2).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/data_structures/skip_list.rs	93.75%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #950      +/-   ##
==========================================
- Coverage   95.44%   95.42%   -0.02%     
==========================================
  Files         332      333       +1     
  Lines       21496    21688     +192     
==========================================
+ Hits        20516    20696     +180     
- Misses        980      992      +12     

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