Support unwrapping and make virtualized Rows work again
Finish renaming transform to convert
Finish unit test for filterRowsBy
Address comments from @adamklein

 - The lazy value was not needed.

 - I figured out how to reused the same `RowSeries` type for both
   `df.Rows` and `df.RowsDense` and I cleaned up and commented the relevant
   parts of the code.