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

Author: wesm

cpp/src/arrow/buffer.cc

@@ -27,11 +27,9 @@
 
 namespace arrow {
 
-Buffer::Buffer(const std::shared_ptr<Buffer>& parent, int64_t offset, int64_t size) {
-  data_ = parent->data() + offset;
-  size_ = size;
+Buffer::Buffer(const std::shared_ptr<Buffer>& parent, int64_t offset, int64_t size)
+    : Buffer(parent->data() + offset, size) {
   parent_ = parent;
-  capacity_ = size;
 }
 
 Buffer::~Buffer() {}