atomic: Add non-member functions for fetch_min and fetch_max

[stotko]

The upcoming C++26 standard will introduce fetch_min and fetch_max functions in the atomic module. We already expose them for atomic and atomic_ref but lack the respective non-member functions. Add an implementation of these and update he signature of the remaining ones to more closely match the C++ standard.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.46%. Comparing base (d621e2b) to head (2676d85).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #443   +/-   ##
=======================================
  Coverage   97.45%   97.46%           
=======================================
  Files          32       32           
  Lines        2594     2602    +8     
=======================================
+ Hits         2528     2536    +8     
  Misses         66       66           

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