split: restore --io-blocksize & remove io::copy

[oech3]

Needed to distinct read/write error in the future and support reflink and zero-copy.

[codspeed-hq]

Merging this PR will not alter performance

✅ 345 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing oech3:split-del-copy (4df1546) with main (f258a3f)}

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. ↩

[oech3]

In the case buffering is not allowed and block size is given, we cannot use Buf{Reader, Writer} in io::copy even reader's error message is hacked to distinct.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/retry (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/misc/write-errors was skipped on 'main' but is now failing.