refactor: Replace gil::detail::copy_n with std::copy_n

[marco-langer]

Description

This PR replaces Gil's gil::detail::copy_n with std::copy_n.

Tasklist

• Ensure all CI builds pass
• Review and approve

[codecov]

Codecov Report

Merging #686 (46497d0) into develop (151fd9c) will decrease coverage by 0.05%.
The diff coverage is 85.71%.

@@             Coverage Diff             @@
##           develop     #686      +/-   ##
===========================================
- Coverage    80.32%   80.27%   -0.06%     
===========================================
  Files          117      117              
  Lines         5032     5019      -13     
===========================================
- Hits          4042     4029      -13     
  Misses         990      990              

[mloskot]

Added line #L244 was not covered by tests

Hmm, this one is interesting. I'd have expected those to be fully covered.

[marco-langer]

I don't see any test cases for core\algorithm\copy_pixels. The other specializations of struct copier_n are apparently triggered by some unrelated test cases from extension or image processing tests AFAIS.

Edit: this is not true, there are some legacy tests, which do not cover this case. However, the Jamfile in test\legacy says these tests should not be refactored or extended. Thus, should we rather add new copy_pixels tests in core\algorithm\copy_pixels.cpp?