Refactor fast factorial algorithm

[bgdnrvsky]

PR Title: Refactor fast factorial algorithm

Description

• Fix typo in variable name
• Use more idiomatic Rust code
• Improve allocation by using Iterator::collect

Type of change

Refactoring

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.
• [ ] 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.
• [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 95.05%. Comparing base (aa8c458) to head (dc03331). Report is 6 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #761 +/- ## ========================================== - Coverage 95.05% 95.05% -0.01% ========================================== Files 306 306 Lines 22736 22734 -2 ========================================== - Hits 21612 21610 -2 Misses 1124 1124 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.