[Feature Request]: Expose pvalue.Row in core.py rather than pvalue.py

### What would you like to happen?

It's semantically confusing that `Row` is defined in `pvalue.py`. `pvalue`s are abstract representations of data while `Row`s are actually concretely instantiated.

It might make more sense to put it in core.py? (with a lingering alias in pvalue.py for backwards compatibility)

### Issue Priority

Priority: 2 (default / most feature requests should be filed as P2)

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