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ARROW-3520: [C++] Add "list_flatten" vector kernel wrapper for Flatten method of ListArray types

This simplifies invocation of this action in bindings.

Closes #7585 from wesm/ARROW-3520

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