Handle unicode characters in duval_algorithm

[realstealthninja]

Fix duval_algorithm.rs on unicode strings being entered

Description

I've replaced all instances of &[u8] with its character equivalent and replaced the other instances
with appropriate substitutes

Related to #683.

Type of change

• Bug fix (non-breaking change which fixes an issue)

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.

note:
cargo clippy does produce errors but none of them were part of the file I was working on. I'd appreciate it if someone took at look at it

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.70%. Comparing base (10c986d) to head (ca2b348).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #685      +/-   ##
==========================================
- Coverage   94.70%   94.70%   -0.01%     
==========================================
  Files         292      292              
  Lines       23292    23287       -5     
==========================================
- Hits        22058    22053       -5     
  Misses       1234     1234              

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[vil02]

I will push one more commit to reduce code duplication in the tests of duval_algorithm.