fix mod.rs formats

[suzp1984]

Fix mod.rs formats

Description

• remove pub for mod sieve_of_eratosthenes in src/math/mod.rs;
• use mod tests for UT in src/string/levenshtein_distance.rs;
• refactor _min3 inside levenshtein_distance;

Type of change

Please delete options that are not relevant.

• Breaking change (fix or feature that would cause existing functionality to not work as expected)

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 checked COUNTRIBUTING.md and my code follows its guidelines.

[siriak]

Looks good, thanks!