fold: read input in chunks

[FidelSch]

This allows processing input that does not contain newlines. Adresses #10227

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/fold/fold-characters. tests/fold/fold-characters is passing on 'main'. Maybe you have to rebase?

[codspeed-hq]

CodSpeed Performance Report

Merging this PR will degrade performance by 5.28%

_{Comparing FidelSch:fold-pipe-flush (a0a006f) with main (cbbff30)}

Summary

⚡ 1 improved benchmark
❌ 1 regressed benchmark
✅ 140 untouched benchmarks
⏩ 180 skipped benchmarks¹

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

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	sort_ascii_utf8_locale	18.1 ms	17.5 ms	+3.84%
❌	Simulation	fold_many_lines[100000]	80.4 ms	84.9 ms	-5.28%

Footnotes

1. 180 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. ↩

[ChrisDryden]

Theres a bug here where non-utf8 characters at the end of a chunk can be split in two so a single multi byte character will be split into multiple characters.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tty/tty-eof (fails in this run but passes in the 'main' branch)

[github-actions]

GNU testsuite comparison:

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)