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ARROW-9145: [C++] Implement BooleanArray::true_count and false_count, add Python bindings

This seemed like a reasonable place to put this, and it seems like it may come in handy.

Closes #7463 from wesm/ARROW-9145

Authored-by: Wes McKinney <wesm@apache.org>
Signed-off-by: Wes McKinney <wesm@apache.org>