[C++] Introduce initial ArraySpan, ExecSpan non-owning / shared_ptr-free data structures for kernel execution, refactor scalar kernels

[asfimport]

This is essential to reduce microperformance overhead as has been discussed and investigated many other places. This first stage of work is to remove the use of Datum and ExecBatch from the input side of only scalar kernels, so that we can work toward using span/view data structures as the inputs (and eventually outputs) of all kernels.

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

Related issues:

• [C++] Improve array expression and kernel evaluation performance on small inputs (is a child of)
• [C++] arrow::compute::CallFunction needs a batch length for nullary functions (is related to)
• [C++] Migrate VectorKernel implementations to use ExecSpan, ArraySpan (is related to)

PRs and other links:

• GitHub Pull Request #13364

_{Note: This issue was originally created as ARROW-16756. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 13364
#13364