cat: avoid unnecessary allocation

[oech3]

Allocate buffer on heap instead of stack for read()/write() show-path which is unnecessary if splice() fast-path succeed.

$ echo 1 > /tmp/1
> taskset -c 0 hyperfine -N --runs 10000 "/tmp/coreutils/target/release/cat-stack /tmp/1" "target/release/cat-heap /tmp/1"
Benchmark 1: /tmp/coreutils/target/release/cat-stack /tmp/1
  Time (mean ± σ):     921.2 µs ±  84.4 µs    [User: 372.9 µs, System: 443.7 µs]
  Range (min … max):   843.0 µs … 3926.9 µs    10000 runs
Benchmark 2: target/release/cat-heap /tmp/1
  Time (mean ± σ):     908.6 µs ± 117.0 µs    [User: 380.6 µs, System: 424.1 µs]
  Range (min … max):   821.4 µs … 4337.6 µs    10000 runs 
Summary
  target/release/cat-heap /tmp/1 ran
    1.01 ± 0.16 times faster than /tmp/coreutils/target/release/cat-stack /tmp/1

related #10832

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/basenc/bounded-memory is now being skipped but was previously passing.
Note: The gnu test tests/dd/no-allocate is now being skipped but was previously passing.
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.
Congrats! The gnu test tests/cut/bounded-memory is now passing!

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/resolution (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/cut/cut-huge-range is now passing!

[oech3]

hyperfine is flakey

[xtqqczze]

Switching from a stack allocation to a heap allocation doesn’t avoid allocation...

[oech3]

buf is declared after splice code.

[oech3]

I saw more perf difference with 1024 * 1024 by switching to vec. So I think vec's allocation is deffered.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails 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.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/cut/cut-huge-range is now passing!