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ARROW-717: [C++] Implement IPC zero-copy round trip for tensors

This patch provides:

```python
WriteTensor(tensor, file, &metadata_length, &body_length));
std::shared_ptr<Tensor> result;
ReadTensor(offset, file, &result));
```

Also implemented `Tensor::Equals` and did some refactoring / code simplification in compare.cc

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

Closes #454 from wesm/ARROW-717 and squashes the following commits:

6c15481 [Wes McKinney] Tensor IPC read/write, and refactoring / code scrubbing