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ARROW-1783: [Python] Provide a "component" dict representation of a serialized Python object with minimal allocation

For systems (like Dask) that prefer to handle their own framed buffer transport, this provides a list of memoryview-compatible objects with minimal copying / allocation from the input data structure, which can similarly be zero-copy reconstructed to the original object.

To motivate the use case, consider a dict of ndarrays:

```
data = {i: np.random.randn(1000, 1000) for i in range(50)}
```

Here, we have:

```
>>> %timeit serialized = pa.serialize(data)
52.7 µs ± 1.01 µs per loop (mean ± std. dev. of 7 runs, 10000 loops each)
```

This is about 400MB of data. Some systems may not want to double memory by assembling this into a single large buffer, like with the `to_buffer` method:

```
>>> written = serialized.to_buffer()
>>> written.size
400015456
```

We provide a `to_components` method which contains a dict with a `'data'` field containing a list of `pyarrow.Buffer` objects. This can be converted back to the original Python object using `pyarrow.deserialize_components`:

```
>>> %timeit components = serialized.to_components()
73.8 µs ± 812 ns per loop (mean ± std. dev. of 7 runs, 10000 loops each)

>>> list(components.keys())
['num_buffers', 'data', 'num_tensors']

>>> len(components['data'])
101

>>> type(components['data'][0])
pyarrow.lib.Buffer
```

and

```
>>> %timeit recons = pa.deserialize_components(components)
93.6 µs ± 260 ns per loop (mean ± std. dev. of 7 runs, 10000 loops each)
```

The reason there are 101 data components (1 + 2 * 50) is that:

* 1 buffer for the serialized Union stream representing the object
* 2 buffers for each of the tensors: 1 for the metadata and 1 for the tensor body. The body is separate so that this is zero-copy from the input

Next step after this is ARROW-1784 which is to transport a pandas.DataFrame using this mechanism

cc @pitrou @jcrist @mrocklin

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

Closes #1362 from wesm/ARROW-1783 and squashes the following commits:

4ec5a89f [Wes McKinney] Add missing decref on error
e8c76d42 [Wes McKinney] Acquire GIL in GetSerializedFromComponents
1d2e0e27 [Wes McKinney] Fix function documentation
fffc7bb6 [Wes McKinney] Typos, add deserialize_components to API
50d2fee5 [Wes McKinney] Finish componentwise serialization roundtrip
58174dde [Wes McKinney] More progress, stubs for reconstruction
b1e31a34 [Wes McKinney] Draft GetTensorMessage
337e1d29 [Wes McKinney] Draft SerializedPyObject::GetComponents
598ef335 [Wes McKinney] Tweak