[C++] Refactor StringTransform

[asfimport]

In compute/kernels/scalar_string.cc}, currently there is a non-trivial inheritance relationship between {{StringTransform and its derived implementation classes. Also, the derived classes have to be templated on the actual string type even though their code doesn't care, making generated code size probably bigger than it should be.

Instead, there could be a StringTransformExec class that delegates to an independent class for per-kernel specifics (mostly: the MaxCodeunits and Transform methods).

Reporter: Antoine Pitrou / @pitrou
Assignee: Antoine Pitrou / @pitrou

PRs and other links:

• GitHub Pull Request #10496

_{Note: This issue was originally created as ARROW-12951. Please see the migration documentation for further details.}

[asfimport]

David Li / @lidavidm:
Is this the case? MaxCodeunits and Transform do depend on the concrete type since they use offset_type. And while you could abstract between the large and regular types, that would preclude kernels specializing on binary vs string types (though admittedly, no kernel does that).

[asfimport]

Antoine Pitrou / @pitrou:
They only depend superficially on the offset type. In practice they could take int64_t always.

[asfimport]

Antoine Pitrou / @pitrou:
Hmm, currently I'm working on a draft for this, though we may end up on different approaches :)

[asfimport]

David Li / @lidavidm:
Ah sorry! In that case please continue, I was just searching through the backlog.

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 10496
#10496