feat: clean up Row class (#215)

### Summary of Changes

* Make `Row` a subclass of `Mapping`
* Give `Row` a usable constructor
* Rename `get_column_names` to `column_names` and make it a property
* Rename `count` to `n_columns` and make it a property
* Rename `get_type_of_column` to `get_column_type`

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Co-authored-by: megalinter-bot <129584137+megalinter-bot@users.noreply.github.com>