[C++] Unify KeyColumnArray and ArraySpan

[asfimport]

Both of these are essentially non-owning views into ArrayData. They were developed somewhat independently but share a pretty similar structure. I don't think we need both and we should unify on a common type for simplicity provided we can show no real performance difference.

Reporter: Weston Pace / @westonpace

Related issues:

• [C++][Compute] Add scalar_hash function (relates to)

_{Note: This issue was originally created as ARROW-17257. Please see the migration documentation for further details.}

[asfimport]

Aldrin Montana / @drin:
Linking ARROW-8991, because I am exploring the use of KeyColumnArray to leverage hashing code from key_hash.h. But, I am also trying to use ExecSpan and ArraySpan which is being added to the kernel execution framework.

[asfimport]

Aldrin Montana / @drin:
Discussed initial directions of this with @save-buffer and @michalursa.

In general, KeyColumnArray tries to minimize it's memory footprint as much as possible, whereas ArraySpan tries to be a non-owning version of ArrayData.

Work in ARROW-8991 is going to take the following direction: KeyColumnArray is essentially a flattened ArraySpan, so when using the Hashing32 and Hashing64 functions, the a KeyColumnArray will be constructed from an ArraySpan.

Otherwise, the 2 classes are very similar and it's okay if ArraySpan eventually replaces/subsumes KeyColumnArray as long as performance regressions are not introduced.

[github-actions]

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