[C++] Process data buffers in batch in ascii_lower / ascii_upper kernels rather than using string_view value iteration

[asfimport]

Also add a benchmark

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

PRs and other links:

• GitHub Pull Request #7418

_{Note: This issue was originally created as ARROW-9115. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 7418
#7418