return Adbc.Column (#70)

* renamed `Adbc.Buffer` to `Adbc.Column`

* wrap primivitive data types in `Adbc.Column`

* wrap list type in `Adbc.Column`

* return metadata if any

* return nullable value in `Adbc.Column`

* implemented `Adbc.Result.to_map/1` and added some tests for it

* updated inline docs for some functions in `Adbc.Column`

* mix format

* use `Enum.zip_with/2`

* added support for `date{32,64}` when binding parameters

* Apply suggestions from code review

Co-authored-by: José Valim <[email protected]>

* remove unused commented code

* allow to use seconds and milliseconds in `Adbc.Buffer.date{32,64}` respectively

* fixed tests related to `Adbc.Result.to_map/1`

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Co-authored-by: José Valim <[email protected]>