Add Euler's Totient function implementation

[allansomensi]

Adds the implementation of the Euler's Totient function with a unit test.

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.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.55172% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.49%. Comparing base (b4aecf4) to head (2c5be4a).

Files with missing lines	Patch %	Lines
src/math/euler_totient.rs	96.55%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #875   +/-   ##
=======================================
  Coverage   95.49%   95.49%           
=======================================
  Files         316      317    +1     
  Lines       22919    22948   +29     
=======================================
+ Hits        21887    21915   +28     
- Misses       1032     1033    +1     

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