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ARROW-9017: [C++][Python] Refactor scalar bindings

TODOs:
 - [x] split PRs into two, one with simplified python to arrow conversions with benchmarks
 - [x] implement union scalar on the C++ side
 - [x] store the index value for dictionary scalar
 - [x] more tests

Closes ARROW-9017, ARROW-9153

Closes #7519 from kszucs/ARROW-9153

Authored-by: Krisztián Szűcs <szucs.krisztian@gmail.com>
Signed-off-by: Wes McKinney <wesm@apache.org>