factor: remove unsafe str::from_utf8_unchecked

[oech3]

No description provided.

[codspeed-hq]

Merging this PR will not alter performance

✅ 311 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing oech3:factor-unsafe (6fe8c3e) with main (c23dc67)²}

Footnotes

1. 46 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (f7ba1f9) during the generation of this report, so c23dc67 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/printf/printf-surprise is now passing!
Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.

[cakebaker]

Thanks!