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ARROW-182: [C++] Factor out Array::Validate into a separate function

This makes it easier to use templates for writing more validation logic, and also we don't need the additional array methods

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

Closes #692 from wesm/ARROW-182 and squashes the following commits:

9753a59 [Wes McKinney] Factor out Array::Validate into a separate function