Implement n bits gray code

[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 95.34%. Comparing base (84c4665) to head (0b0e30b).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #787      +/-   ##
==========================================
+ Coverage   95.09%   95.34%   +0.24%     
==========================================
  Files         309      310       +1     
  Lines       22859    22689     -170     
==========================================
- Hits        21738    21632     -106     
+ Misses       1121     1057      -64     

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[sozelfist]

Can you have a look, @vil02?