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ARROW-7784: [C++] Improve compilation time of arrow/array/diff.cc and reduce code size

This refactors diff.cc slightly to instantiate fewer templates.

On my machine with clang-8:

* Before: 15.5s compilation time of diff.cc in -03, 1633704 bytes of object code
* After: 4.1s compilation time, 498328 bytes of object code

There are probably more improvements here both in compilation time and code size but cutting 10 seconds out of release builds is already a good improvement.

Closes #7311 from wesm/diff-compile-faster

Authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>