split: preliminary for distinguishing read/write errors (& reflink)

[oech3]

GNU shows file path which caused I/O error. We need to separate out read/write instead of using std::io::copy to do same thing. Add copy method.
(This refactoring is useful for adding reflink support in the future BTW...)

[codspeed-hq]

Merging this PR will improve performance by 9.76%

⚡ 1 improved benchmark
✅ 322 untouched benchmarks
⏩ 46 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	split_bytes	311.7 µs	284 µs	+9.76%

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 (efdb4c1) with main (bc24d39)²}

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. ↩

2. No successful run was found on main (4bc3b68) during the generation of this report, so bc24d39 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/inotify-dir-recreate (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/symlink (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/tail/pipe-f is now being skipped but was previously passing.

[oech3]

While CodSpeed reported performance gain, this is double buffered by BufReader. I'll fix it at different PR.