Port the reporter from using std::ostream to FILE*

[Neverlord]

Same motivation as https://github.com/actor-framework/actor-framework/pull/2198: we want to move away from std::ostream & friends.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 71.94%. Comparing base (98acd6f) to head (d50c922).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2199      +/-   ##
==========================================
+ Coverage   71.89%   71.94%   +0.04%     
==========================================
  Files         632      632              
  Lines       30601    30600       -1     
  Branches     3334     3333       -1     
==========================================
+ Hits        22002    22015      +13     
+ Misses       6686     6674      -12     
+ Partials     1913     1911       -2     

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