mv: use copy_stream for fast cross-device move

[oech3]

copy_stream has splice path and normal io::copy has some unnecessary syscall failures e.g. copy_file_range which fails at most cases with cross-device.

> hyperfine --ignore-failure "target/release/mv-main /lib/chromium/chromium /tmp/c" "target/release/mv-s /lib/chromium/chromium /tmp/c"
Benchmark 1: target/release/mv-main /lib/chromium/chromium /tmp/c
  Time (mean ± σ):      53.4 ms ±   1.4 ms    [User: 0.4 ms, System: 52.7 ms]
  Range (min … max):    51.4 ms …  60.9 ms    57 runs
Benchmark 2: target/release/mv-s /lib/chromium/chromium /tmp/c
  Time (mean ± σ):      45.7 ms ±   0.7 ms    [User: 0.5 ms, System: 45.0 ms]
  Range (min … max):    44.9 ms …  50.6 ms    58 runs
Summary
  target/release/mv-s /lib/chromium/chromium /tmp/c ran
    1.17 ± 0.04 times faster than target/release/mv-main /lib/chromium/chromium /tmp/c

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/tail/pipe-f2. tests/tail/pipe-f2 is passing on 'main'. Maybe you have to rebase?
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/rm/many-dir-entries-vs-OOM is now being skipped but was previously passing.
Skip an intermittent issue tests/pr/bounded-memory (was skipped on 'main', now failing)

[cakebaker]

Thanks!