Combine self-hosted coverage with clean codecov

[juntyr]

Follow-up to #483 - all we needed was cargo clean ... and two hours of debugging

* [ ] I've included my change in CHANGELOG.md

[codecov-commenter]

Codecov Report

Patch coverage: 99.29% and project coverage change: +12.44% 🎉

Comparison is base (14734c1) 82.83% compared to head (8a9a545) 95.27%.
Report is 3 commits behind head on master.

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Additional details and impacted files

@@             Coverage Diff             @@
##           master     #486       +/-   ##
===========================================
+ Coverage   82.83%   95.27%   +12.44%     
===========================================
  Files          74       74               
  Lines        9281     8852      -429     
===========================================
+ Hits         7688     8434      +746     
+ Misses       1593      418     -1175     

Files Changed	Coverage Δ
src/error.rs	42.91% <25.00%> (-0.36%)	⬇️
src/parse.rs	96.61% <99.27%> (+21.29%)	⬆️
tests/481_number_underscores_suffixes.rs	99.79% <99.79%> (ø)
src/de/mod.rs	96.55% <100.00%> (+21.64%)	⬆️
src/ser/mod.rs	96.59% <100.00%> (+25.02%)	⬆️
src/ser/tests.rs	100.00% <100.00%> (+13.69%)	⬆️
tests/370_float_parsing.rs	100.00% <100.00%> (+1.78%)	⬆️

... and 48 files with indirect coverage changes

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