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ARROW-1886: [C++/Python] Flatten struct columns in table

Add C++ and Python APIs to flatten struct fields and struct columns.

Based on PR #1755.

Author: Antoine Pitrou <antoine@python.org>

Closes #1768 from pitrou/ARROW-1886-flatten-table and squashes the following commits:

a821b77 <Antoine Pitrou> Add test for empty column
b8335af <Antoine Pitrou> ARROW-1886:  Flatten struct columns in table