[C++] Add hash kernel benchmarks, investigate faster alternative non-SIMD hash functions

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Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

_{Note: This issue was originally created as ARROW-1823. Please see the migration documentation for further details.}

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Antoine Pitrou / @pitrou:
Hash-kernel benchmarks already exist in compute-benchmark.cc, I'm not sure more are needed?

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Wes McKinney / @wesm:
Closing. Your hashing PR today addresses my goal here (to improve the performance of hashing)