https://issues.apache.org/jira/browse/ARROW-12946

LGTM. Need to add implementation for UTF8.

@lidavidm please enable CI

For CI, it appears that Ubuntu 20.04 provides utf8proc 2.5, but the isupper/islower functions are not provided until 2.6: https://juliastrings.github.io/utf8proc/releases/

Meanwhile, RTools35 is using utf8proc 2.4; it builds utf8proc from source due to some other issue, but I believe it's using the system headers anyways looking at the compiler command since the system headers come before the self-built utf8proc ones.

I think bumping the minimum utf8proc version and fiddling with the build flags so that the utf8proc headers take precedence will be needed.

An alternative solution is to use helper functions already available, refer to https://github.com/apache/arrow/blob/master/cpp/src/arrow/compute/kernels/scalar_string.cc#L1391-L1408

Related notes in Arrow w.r.t. to lower/upper casing of UTF8 can be found in:

• https://github.com/apache/arrow/blob/master/python/pyarrow/tests/test_compute.py#L668-L693
• https://github.com/apache/arrow/blob/master/cpp/src/arrow/compute/kernels/scalar_string_test.cc#L578-L601

Meanwhile, RTools35 is using utf8proc 2.4; it builds utf8proc from source due to some other issue, but I believe it's using the system headers anyways looking at the compiler command since the system headers come before the self-built utf8proc ones.

Yeah the logs say

-- Could NOT find utf8proc (missing: utf8proc_LIB) (found suitable version "2.4.0", minimum required is "2.2.0")

which doesn't make sense.

As @edponce suggested, I replaced some utf8proc functions with helper functions. After all the CI passes. This PR is ready for review.

Some tests invoke the incorrect kernel (ASCII test uses utf8_capitalize and/or viceversa). These "typos" are fixed in #10869