Replace CAtom's bitfield with packed struct

[frmdstryr]

Reduce manual bit fiddling...

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.67%. Comparing base (6b689f0) to head (969291a).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #243   +/-   ##
=======================================
  Coverage   97.67%   97.67%           
=======================================
  Files          24       24           
  Lines        1074     1074           
  Branches      162      162           
=======================================
  Hits         1049     1049           
  Misses         12       12           
  Partials       13       13           

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[frmdstryr]

I made one more micro optimization.... In CAtom_new it currently calls atom->set_notifications_enabled( true );, instead of doing a second write (since it rewrites when setting slots), I inverted the meaning of the notifications bit so it is on by default.

I haven't looked if the compiler is smart enough to optimize it into a single write or not but this way it should only ever be one write.

Edit: I was curious

Current behavior: https://godbolt.org/z/P9zK9eEbK
Inverting logic: https://godbolt.org/z/zexx8rrnn

so it removes a single 'or' instruction