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 not alter performance

✅ 323 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing oech3:split-unbuf (263b158) with main (43ef8d0)}

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. ↩

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/rm/many-dir-entries-vs-OOM is now being skipped but was previously passing.
Note: The gnu test tests/misc/write-errors was skipped on 'main' but is now failing.

[oech3]

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