sagemathgh-38392: get rid of sage_eval in grobner_fan

    
parsing the input by hand instead of using the big hammer `sage_eval`

### 📝 Checklist

- [x] The title is concise and informative.
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- [x] I have created tests covering the changes.
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preview.
    
URL: sagemath#38392
Reported by: Frédéric Chapoton
Reviewer(s): Matthias Köppe