Implement subset sum using backtracking

[sozelfist]

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.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.05%. Comparing base (46ad2da) to head (192d3b6).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #765   +/-   ##
=======================================
  Coverage   95.05%   95.05%           
=======================================
  Files         307      308    +1     
  Lines       22729    22749   +20     
=======================================
+ Hits        21605    21625   +20     
  Misses       1124     1124           

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[sozelfist]

Can you review this PR, @vil02?