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ARROW-782: [C++] API cleanup, change public member access in DataType classes to functions, use class instead of struct

Breaking APIs isn't ideal, but this one is fairly long overdue. The DataType classes are more than passive data carriers, and Google's C++ guide recommends using class instead of struct for this. That means we should put members in protected or private scope, and access them.

I also renamed a couple of things to help with code clarity

* `DataType::type` is now `DataType::id()`
* `Array::type_enum` is not `Array::type_id`

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

Closes #520 from wesm/ARROW-782 and squashes the following commits:

f8dd131 [Wes McKinney] Revert changes with garrow_data_type_new_raw
40de60e [Wes McKinney] Fix glib usages of changed APIs
0097122 [Wes McKinney] Update post rebase
f725655 [Wes McKinney] cpplint
e77f6a5 [Wes McKinney] Change public member access in DataType classes to functions, use class instead of struct