Ascii/unicode functions

[asfimport]

Implement the Postgres Ascii/Unicode functions

Reporter: Mike Seddon / @seddonm1
Assignee: Mike Seddon / @seddonm1

PRs and other links:

• GitHub Pull Request #9625

_{Note: This issue was originally created as ARROW-11653. Please see the migration documentation for further details.}

[asfimport]

Andrew Lamb / @alamb:
Issue resolved by pull request 9625
#9625