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ARROW-4095: [C++] Optimize DictionaryArray::Transpose() for trivial transpositions

Note this doesn't optimize the case of a trivial transposition but where the index type changes.

Also fix handling of sliced arrays, and add tests.

Closes #5230 from pitrou/ARROW-4095-dictarray-transpose-optimization and squashes the following commits:

acae5e0c6 <Antoine Pitrou> ARROW-4095:  Optimize DictionaryArray::Transpose() for trivial transpositions

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Micah Kornfield <emkornfield@gmail.com>