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ARROW-14174: [C++] Deduplicate some Decimal/FixedSizeBinary kernels

Now that their respective scalar classes have a common `util::string_view view() const` getter, we can use that to consolidate some kernel implementations and hopefully shave binary size/build time a little.

Closes #11536 from lidavidm/arrow-14174

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>