numfmt & others: Dedup some code

[sylvestre]

No description provided.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/resolution (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/inotify-dir-recreate (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)

[codspeed-hq]

Merging this PR will degrade performance by 4.82%

❌ 8 regressed benchmarks
✅ 301 untouched benchmarks
⏩ 46 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	numfmt_to_iec[10000]	35.7 ms	37.5 ms	-4.81%
❌	Simulation	numfmt_large_numbers_si[10000]	38.4 ms	40.2 ms	-4.43%
❌	Simulation	numfmt_to_si_precision[10000]	37 ms	38.8 ms	-4.63%
❌	Simulation	numfmt_round_modes[("up", 10000)]	36.3 ms	38.1 ms	-4.71%
❌	Simulation	numfmt_round_modes[("down", 10000)]	36.2 ms	38 ms	-4.74%
❌	Simulation	numfmt_round_modes[("towards-zero", 10000)]	36.3 ms	38.1 ms	-4.73%
❌	Simulation	numfmt_padding[(10000, 50)]	38.8 ms	40.6 ms	-4.48%
❌	Simulation	numfmt_to_si[10000]	35.5 ms	37.3 ms	-4.82%

---

_{Comparing sylvestre:numfmt-3 (96be503) with main (c5cff82)}

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. ↩

[cakebaker]

What's the reason for the renames?

[cakebaker]

This feels error prone, though currently I don't see a better solution :|

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/timeout/timeout-group. tests/timeout/timeout-group is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/seq/seq-epipe is now being skipped but was previously passing.
Congrats! The gnu test tests/printf/printf-surprise is now passing!