ARROW-12714: [C++] String title case kernel

[edponce]

This PR adds scalar string compute functions for titlecasing a string, namely "ascii_title" and "utf8_title". Simple titlecasing is performed, only every cased character following an uncased character is uppercased.
Additional changes included with this PR are:

• restructure StringTransformCodepointXXX classes to support vector string kernels using codepoint transforms
• update capitalize kernels

[github-actions]

https://issues.apache.org/jira/browse/ARROW-12714

[pitrou]

@edponce Do you know when this will be ready for review? Or do you need help on this?

[edponce]

@pitrou I am working on completing this PR today and would greatly appreciate your review.

[edponce]

The capitalize and title kernels are the first vector string kernels that perform code point transforms. The code point transforms (case changes) can grow in bytes and thus required the use of MaxCodeUnits with the 3/2 growth factor. Also, these kernels depend on EnsureLookupTablesFilled to be called in PreExec method. These two requirements existed in different classes (CaseMappingTransform and StringTransformCodepoint) and their use applied TransformCodepoint (scalar method) instead of Transform (custom). For these reasons, I created StringTransformCodepointBase class with the growth factor and lookup tables methods. Now the available classes can be chosen depending on kind of kernel and for those that apply codepoint transforms.

cc @pitrou

[pitrou]

Thanks for doing this!

[edponce]

@ianmcook Could you revise the R binding for the titlecase kernel? stringr has a str_to_title function.

[pitrou]

LGTM, just a couple more questions / comments

[pitrou]

Just one question, you may or may not want to act on it.
However, can you fix the lint failure? archery lint --clang-format should do it.

[pitrou]

Thank you very much @edponce !