Further flatten fuzzer fixes

[juntyr]

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

This PR is merged early to fix some long-time outstanding fuzzer-found issues. Most bugs are with serde (flattened fields in particular) and our fuzzer is adapted to not trigger them.

This PR should also improve one nasty edge case of parsing complexity.

• [ ] Follow-up this PR with more tests and document newly found restrictions precisely -> #524

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (d509208) 100.00% compared to head (8fe8188) 100.00%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #523 +/- ## ========================================== Coverage 100.00% 100.00% ========================================== Files 83 83 Lines 14443 14588 +145 ========================================== + Hits 14443 14588 +145 ```

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