split: remove buffering

[oech3]

Closes #13068
I have no idea for tests.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/io-errors (fails in this run but passes in the 'main' branch)

[codspeed-hq]

Merging this PR will improve performance by 10.67%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 3 improved benchmarks
✅ 320 untouched benchmarks
⏩ 46 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	split_bytes	390 µs	324.8 µs	+20.1%
⚡	Memory	split_number_chunks	941.3 KB	860.8 KB	+9.35%
⚡	Simulation	split_number_chunks	142.4 µs	138 µs	+3.21%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing oech3:split-unbuf (d611474) with main (be93dc3)}

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]

Thanks!