refactor: move main merge logic from df to reshape package #2217
      
        
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Reason for this refactor:
The
dataframefile is already super long. We'd better not add more stuff there. Plus, this structure reflect pandas more: https://github.com/pandas-dev/pandas/blob/9c8bc3e55188c8aff37207a74f1dd144980b8874/pandas/core/frame.py#L10842-L10873b/445802345 🦕