Fixes and type stability improvements

[codecov[bot]]

Codecov Report

Patch coverage: 94.11% and project coverage change: +1.10% :tada:

Comparison is base (38e32f0) 76.96% compared to head (1dd52be) 78.06%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #7 +/- ## ========================================== + Coverage 76.96% 78.06% +1.10% ========================================== Files 11 11 Lines 395 424 +29 ========================================== + Hits 304 331 +27 - Misses 91 93 +2 ``` | [Files Changed](https://app.codecov.io/gh/oschulz/AutoDiffOperators.jl/pull/7?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Oliver+Schulz) | Coverage Δ | | |---|---|---| | [ext/AutoDiffOperatorsEnzymeExt.jl](https://app.codecov.io/gh/oschulz/AutoDiffOperators.jl/pull/7?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Oliver+Schulz#diff-ZXh0L0F1dG9EaWZmT3BlcmF0b3JzRW56eW1lRXh0Lmps) | `97.43% <87.50%> (-2.57%)` | :arrow_down: | | [ext/AutoDiffOperatorsZygoteExt.jl](https://app.codecov.io/gh/oschulz/AutoDiffOperators.jl/pull/7?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Oliver+Schulz#diff-ZXh0L0F1dG9EaWZmT3BlcmF0b3JzWnlnb3RlRXh0Lmps) | `88.37% <95.00%> (+3.75%)` | :arrow_up: | | [ext/AutoDiffOperatorsForwardDiffExt.jl](https://app.codecov.io/gh/oschulz/AutoDiffOperators.jl/pull/7?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Oliver+Schulz#diff-ZXh0L0F1dG9EaWZmT3BlcmF0b3JzRm9yd2FyZERpZmZFeHQuamw=) | `77.50% <100.00%> (+3.21%)` | :arrow_up: |

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