Atomic cleanup and maintainence

[jxy-s]

Cleans up atomic header to be more friendly to read. Eliminates large blocks of ifdefs and opts to make the conditional up-front.

Choice is made here to require explicit atomic ordering by default.

Also contains miscellaneous maintenance tasks for tooling and compatibility.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.94%. Comparing base (a877d5a) to head (4135e9a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #23      +/-   ##
==========================================
- Coverage   91.05%   90.94%   -0.11%     
==========================================
  Files          17       17              
  Lines        1934     1922      -12     
  Branches      236      236              
==========================================
- Hits         1761     1748      -13     
+ Misses         14       13       -1     
- Partials      159      161       +2     

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