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ARROW-12835: [C++][Python][R] Implement case-insensitive match using RE2

This uses RE2 to implement a case-insensitive substring search.

Originally, I implemented this using utf8proc, but then found it was about an order of magnitude slower than RE2. (This isn't an apples-to-apples comparison; utf8proc does it more 'properly' and handles more Unicode corners.) So I switched to just doing it with RE2 instead, especially since the utf8proc approach was complicated. (You can still see it in the original commit here if you're curious.)

Closes #10369 from lidavidm/arrow-12835

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>