Update naming of bool template parameters, move internal methods to private

[westonpace]

This is just a suggestion. It was easier typing this up than trying to communicate everything in a PR comment. Feel free to reject.

I was still having trouble compiling because there were a few places using combine_hashes (I think this is probably a bug in clang because it goes away in clang-11).

Before I suggested COMBINE_HASHES_T but, in retrospect, _T doesn't make sense as it isn't a type. Furthermore, it seems we have some inconsistency between template <bool foo>, template <bool FOO>, and template <bool kFoo> (all three are used in the code base). Based on an old ML discussion I think kFoo is correct so I am suggesting that here. Feel free to pick another style if that appeals to you.

Also, I had been playing around with making hash_fixed and hash_varlen private. This seems to work ok but feel free to ignore if there is a reason these need to be public.

[github-actions]

Thanks for opening a pull request!

If this is not a minor PR. Could you open an issue for this pull request on JIRA? https://issues.apache.org/jira/browse/ARROW

Opening JIRAs ahead of time contributes to the Openness of the Apache Arrow project.

Then could you also rename pull request title in the following format?

ARROW-${JIRA_ID}: [${COMPONENT}] ${SUMMARY}

or

MINOR: [${COMPONENT}] ${SUMMARY}

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• Other pull requests
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