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ARROW-1235: [C++] Make operator<< for Array/Status and std::ostream inline

I'm unable to reproduce the linker failure, but I think this will fix it.

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

Closes #864 from wesm/xcode-6.4-fixes and squashes the following commits:

43bb0b8c [Wes McKinney] Add tests for operator<< with Status and Array
f7524379 [Wes McKinney] Also make operator<<(std::ostream& for Array inline, add Array::ToString
cf2b4719 [Wes McKinney] Make operator<< for Status inline