Add private constructors for stateless marshalers

[laurit]

Skipped the ones that are modified in #6429

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.85%. Comparing base (36bc703) to head (47fb68d).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6434      +/-   ##
============================================
- Coverage     90.86%   90.85%   -0.01%     
+ Complexity     6030     6020      -10     
============================================
  Files           651      651              
  Lines         17728    17718      -10     
  Branches       1777     1777              
============================================
- Hits          16108    16098      -10     
  Misses         1105     1105              
  Partials        515      515              

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[jack-berg]

Thanks for following up on this!