Improve infix to postfix notation algorithm

[bgdnrvsky]

Description

• Moved the algorithm to a more appropriate place
• Introduced the error handling for the algorithm
• Added erroneous unit tests

Type of change

Please delete options that are not relevant.

• 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 97.72727% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.08%. Comparing base (b8fa915) to head (c61e3af).

Files	Patch %	Lines
src/math/infix_to_postfix.rs	97.72%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #772      +/-   ##
==========================================
+ Coverage   95.06%   95.08%   +0.01%     
==========================================
  Files         309      309              
  Lines       22792    22789       -3     
==========================================
  Hits        21668    21668              
+ Misses       1124     1121       -3     

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[vil02]

There was an update of clippy and some warnings become active. Could you just suppress them in Cargo.toml? Done in c61e3af.