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ARROW-1567: [C++] Implement "fill_null" function that replaces null values with a scalar value

This PR implements fill null for most primitive types.

Please note this is my first time contribiting to such a project. I've attempted to follow all available guidelines.

There also may be obvious implementation details missing, as I am relatively new to CPP and it's intricacies.

Closes #7635 from c-jamie/ARROW-1587-implement-fill-null

Lead-authored-by: c-jamie <jamie.b.clery@gmail.com>
Co-authored-by: Wes McKinney <wesm@apache.org>
Signed-off-by: Wes McKinney <wesm@apache.org>