API: Use lowest precision floating type found in reindex with columns
- [ ] closes #51059 (Replace xxxx with the GitHub issue number)
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The problem there was that the ``.values`` call here
https://github.com/pandas-dev/pandas/blob/a131266c8ab7c83f73331b5be5ea49d9a628f075/pandas/core/reshape/reshape.py#L746
upcasts the values, so the float32 will become float64.

I had to modify reindex in order to make this possible, since I don't think you can easily tell whether an all-nan col was there before or was created because of re-indexing. 
   - I guess could modify stack, but then I'd be duplicating reindex logic.

I think I overcomplicated this.