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ARROW-5531: [Python] Implement Array.from_buffers for varbinary and nested types, add DataType.num_buffers property

Thanks to Antoine's recent work on `Array::View` this method can be made more robust and safe by checking for the correct number of buffers.

Author: Wes McKinney <wesm+git@apache.org>

Closes #4537 from wesm/ARROW-5531 and squashes the following commits:

ec0695d86 <Wes McKinney> Address code review feedback
a72533831 <Wes McKinney> Implement Array.from_buffers for nested types, add DataType.num_buffers, more checks