tac: replace io::copy with fast_copy

[oech3]

No description provided.

[github-actions]

Binary size comparison:

Individual binary size comparison VS main (threshold: >=5% AND >=4 KB).

Total size of compared binaries: 148.57 MB (+972 KB, +0.64%)

Significant per-binary changes:
  ls         2.11 MB ->    3.25 MB  (+1.14 MB, +54.36%)
  mkfifo     1.16 MB ->    1.08 MB  (-76 KB, -6.42%)
  mknod      1.16 MB ->    1.09 MB  (-76 KB, -6.38%)
  mkdir      1.16 MB ->    1.09 MB  (-72 KB, -6.04%)

[github-actions]

GNU testsuite comparison:

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

[sylvestre]

thanks