arrow

ARROW-13495: [C++][Compute] Fixing unaligned memory access in GrouperFastImpl

Temp vectors used in grouper implementation were allocated using stack implementation.
The buffer addresses returned by the stack were not aligned to the size of requested vector element.
Changing this to have returned buffers aligned to 8 byte boundaries.

Closes #10835 from michalursa/ARROW-13495-fix-alignment-for-temp-vectors

Authored-by: michalursa <michal@ursacomputing.com>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>