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ARROW-1904: [C++] Deprecate PrimitiveArray::raw_values

I was mistaken about this method's handling of the offset parameter, but it doesn't work correctly for boolean data (also a subclass of `PrimitiveArray`), so I think it's better to remove this method altogether

Author: Wes McKinney <wes.mckinney@twosigma.com>

Closes #1409 from wesm/ARROW-1904 and squashes the following commits:

eb735555 [Wes McKinney] clang-format
fa88b4c9 [Wes McKinney] Deprecate PrimitiveArray::raw_values, add implementation for FixedSizeBinary