pr: simplify the implementation of reading pages

[jfinkels]

Simplify the implementation of reading pages by using loops instead of complex iterators. This allows us to scan for both newline and form feed characters in a straightforward way (using memchr).

The current implementation of pr has very different behavior from GNU pr. This pull request is preparation to make some (hopefully many) further improvements.

[codspeed-hq]

Merging this PR will degrade performance by 26.25%

❌ 7 regressed benchmarks
✅ 275 untouched benchmarks
⏩ 38 skipped benchmarks¹

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

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Memory	sort_key_field[500000]	45.8 MB	51.8 MB	-11.46%
❌	Memory	sort_ascii_only[500000]	22.2 MB	28.3 MB	-21.74%
❌	Memory	sort_long_line[160000]	724.4 KB	982.3 KB	-26.25%
❌	Memory	sort_accented_data[500000]	22.1 MB	28.3 MB	-21.78%
❌	Memory	sort_mixed_data[500000]	22.5 MB	27.2 MB	-17.15%
❌	Memory	cp_large_file[16]	107.6 KB	118 KB	-8.83%
❌	Simulation	cp_large_file[16]	369.8 µs	384.6 µs	-3.84%

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_{Comparing jfinkels:pr-simplify (00ce6ba) with main (799de29)}

Footnotes

1. 38 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. ↩

[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)

[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)