make element-type of `zero_ket` as complex

[ytdHuang]

As title, because usually people will make a zero ket first and start to stack some values at different position of the vector. It would be better to generate it with the element-type ComplexF64, so that users can directly assign complex values to it.

[codecov[bot]]

Codecov Report

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

Project coverage is 93.20%. Comparing base (1dc43b9) to head (c917a16).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #201 +/- ## ======================================= Coverage 93.20% 93.20% ======================================= Files 28 28 Lines 2150 2150 ======================================= Hits 2004 2004 Misses 146 146 ```

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