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ARROW-7754: [C++] Make Result<> faster

The main culprit for slowness was the underlying variant<> implementation.

The new implementation has comparable speed to returning Status + passing
a result-out pointer parameter.

Closes #6356 from pitrou/ARROW-7754-result-perf and squashes the following commits:

d3fc5456f <Antoine Pitrou> Remove commented out code + add a tests
649541699 <Antoine Pitrou> ARROW-7754:  Make Result<> faster

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>