Implement Isogram

[sozelfist]

Checklist:

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

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.38%. Comparing base (af178ff) to head (589ffd6).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #816   +/-   ##
=======================================
  Coverage   95.38%   95.38%           
=======================================
  Files         311      312    +1     
  Lines       22625    22645   +20     
=======================================
+ Hits        21581    21601   +20     
  Misses       1044     1044           

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[sozelfist]

Can you have a look, @vil02?