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ARROW-14243: [C++] Split vector_sort.cc

Move array sorting to a separate translation unit.
This should make vector_sort.cc slightly less heavy to compile.

Closes #11352 from pitrou/ARROW-14243-split-vector-sort

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