feat: support serializing structs into $value named fields

[Mingun]

Replaces #878, because I cannot push to it

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 41.66667% with 14 lines in your changes missing coverage. Please review.

Project coverage is 55.23%. Comparing base (254fbd2) to head (69631c2).
Report is 54 commits behind head on master.

Files with missing lines	Patch %	Lines
src/de/mod.rs	30.00%	14 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #883      +/-   ##
==========================================
- Coverage   60.74%   55.23%   -5.52%     
==========================================
  Files          41       42       +1     
  Lines       16044    15359     -685     
==========================================
- Hits         9746     8483    -1263     
- Misses       6298     6876     +578     

Flag	Coverage Δ
unittests	55.23% <41.66%> (-5.52%)	⬇️

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