perf(factor): optimize output throughput using itoa

[CrazyRoka]

This PR optimizes the output performance of the factor utility by replaces the write! macro and format! calls with itoa for faster integer-to-string conversion.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/follow-name (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/shuf/shuf-reservoir (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/sort/sort-stale-thread-mem (passes in this run but fails in the 'main' branch)

[sylvestre]

i don't see wins with codspeed
did you run hyperfine ? thanks