arrow

ARROW-2514: [Python] Speed up inferring nested Numpy array

Before:
```
$ python -m timeit -s "import numpy as np, pyarrow as pa; data = [np.arange(10000)]" "pa.array(data)"
100 loops, best of 3: 2.68 msec per loop
```

After:
```
$ python -m timeit -s "import numpy as np, pyarrow as pa; data = [np.arange(10000)]" "pa.array(data)"
1000 loops, best of 3: 1.21 msec per loop
```

Conversion itself is still slow, though.

Author: Antoine Pitrou <antoine@python.org>

Closes #2308 from pitrou/ARROW-2514-infer-nested-np-array and squashes the following commits:

ba95f081 <Antoine Pitrou> ARROW-2514:  Speed up inferring nested Numpy array