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ARROW-6754: [C++] Merge allocator.h into stl.h

Merged files into stl.h and also merged the test file allocator_test.cc into stl_test.cc.

Closes #5560 from david1437/ARROW-6754 and squashes the following commits:

f9c9c98af <Antoine Pitrou> ARROW-6754:  Merge allocator.h into stl.h

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