refactor: copy_from_slice per row

[FreezyLemon]

Found this while working on #51.

Benchmarking copy_from_slice_u8: Collecting 100 samples in estimated 5.0011 s (131k iterati
copy_from_slice_u8      time:   [38.180 µs 38.386 µs 38.580 µs]
                        change: [−95.645% −95.632% −95.620%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 18 outliers among 100 measurements (18.00%)
  4 (4.00%) low severe
  4 (4.00%) high mild
  10 (10.00%) high severe

Benchmarking copy_from_slice_u16: Collecting 100 samples in estimated 5.0325 s (71k iterati
copy_from_slice_u16     time:   [70.285 µs 70.848 µs 71.357 µs]
                        change: [−92.103% −92.069% −92.032%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 20 outliers among 100 measurements (20.00%)
  2 (2.00%) high mild
  18 (18.00%) high severe

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.29%. Comparing base (f629ccb) to head (11e49b3).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #52      +/-   ##
==========================================
+ Coverage   94.27%   94.29%   +0.01%     
==========================================
  Files           3        3              
  Lines         332      333       +1     
==========================================
+ Hits          313      314       +1     
  Misses         19       19              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[shssoichiro]

I'm surprised the compiler wasn't already optimizing this into a memcpy, so good catch.