Support mul! with complex vectors

[codecov[bot]]

Codecov Report

Base: 80.00% // Head: 80.00% // No change to project coverage :thumbsup:

Coverage data is based on head (8911f67) compared to base (e75f3f6). Patch coverage: 81.81% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2 +/- ## ======================================= Coverage 80.00% 80.00% ======================================= Files 2 2 Lines 25 25 ======================================= Hits 20 20 Misses 5 5 ``` | [Impacted Files](https://codecov.io/gh/JuliaApproximation/FastTransformsForwardDiff.jl/pull/2?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation) | Coverage Δ | | |---|---|---| | [src/fft.jl](https://codecov.io/gh/JuliaApproximation/FastTransformsForwardDiff.jl/pull/2/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL2ZmdC5qbA==) | `91.66% <ø> (+4.71%)` | :arrow_up: | | [src/FastTransformsForwardDiff.jl](https://codecov.io/gh/JuliaApproximation/FastTransformsForwardDiff.jl/pull/2/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL0Zhc3RUcmFuc2Zvcm1zRm9yd2FyZERpZmYuamw=) | `69.23% <81.81%> (+69.23%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation)

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