cmp: stop allocating for byte printing

[kov]

This makes verbose comparison of 37MB completely different files 2.34x faster than our own baseline, putting our cmp at almost 6x faster than GNU cmp (/opt/homebrew/bin/cmp) on my M4 Pro Mac. The output remains identical to that of GNU cmp. Mostly equal and smaller files do not regress.

Benchmark 1: ./bin/baseline/diffutils cmp -lb t/huge t/eguh
  Time (mean ± σ):      1.669 s ±  0.011 s    [User: 1.594 s, System: 0.073 s]
  Range (min … max):    1.654 s …  1.689 s    10 runs

  Warning: Ignoring non-zero exit code.

Benchmark 2: ./target/release/diffutils cmp -lb t/huge t/eguh
  Time (mean ± σ):     714.2 ms ±   4.1 ms    [User: 629.3 ms, System: 82.7 ms]
  Range (min … max):   707.2 ms … 721.5 ms    10 runs

  Warning: Ignoring non-zero exit code.

Benchmark 3: /opt/homebrew/bin/cmp -lb t/huge t/eguh
  Time (mean ± σ):      4.213 s ±  0.050 s    [User: 4.128 s, System: 0.081 s]
  Range (min … max):    4.160 s …  4.316 s    10 runs

  Warning: Ignoring non-zero exit code.

Benchmark 4: /usr/bin/cmp -lb t/huge t/eguh
  Time (mean ± σ):      3.892 s ±  0.048 s    [User: 3.819 s, System: 0.070 s]
  Range (min … max):    3.808 s …  3.976 s    10 runs

  Warning: Ignoring non-zero exit code.

Summary
  ./target/release/diffutils cmp -lb t/huge t/eguh ran
    2.34 ± 0.02 times faster than ./bin/baseline/diffutils cmp -lb t/huge t/eguh
    5.45 ± 0.07 times faster than /usr/bin/cmp -lb t/huge t/eguh
    5.90 ± 0.08 times faster than /opt/homebrew/bin/cmp -lb t/huge t/eguh

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.37%. Comparing base (df90e37) to head (3d30641).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #153      +/-   ##
==========================================
+ Coverage   86.35%   86.37%   +0.01%     
==========================================
  Files          13       13              
  Lines        6207     6215       +8     
  Branches      520      515       -5     
==========================================
+ Hits         5360     5368       +8     
  Misses        846      846              
  Partials        1        1              

Flag	Coverage Δ
macos_latest	86.31% <100.00%> (+0.01%)	⬆️
ubuntu_latest	86.40% <100.00%> (+0.01%)	⬆️
windows_latest	20.21% <100.00%> (+0.10%)	⬆️

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[sylvestre]

could use extend_from_slice with a constant array instead of a loop

[sylvestre]

write_visible_byte returns usize, but write_visible_byte_padded returns ()

[sylvestre]

some minor suggestions