utility: Add custom pair implementation

[stotko]

With thrust 2.3.0+ (bundled in CUDA 12.4+), their implementation of pair have been replaced by cuda:std::pair from libcudacxx. Unfortunately, this causes compiler errors due to ambiguous namespace resolution for overlapping functionality between stdgpu and libcudacxx. Replace the alias to thrust's pair class with a custom implementation to fix these issues.

This further reduces the dependency to thrust, see #279.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.38%. Comparing base (1b6a331) to head (6976d5b).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #425      +/-   ##
==========================================
+ Coverage   97.33%   97.38%   +0.05%     
==========================================
  Files          31       32       +1     
  Lines        2512     2524      +12     
==========================================
+ Hits         2445     2458      +13     
+ Misses         67       66       -1     

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