Use STRING_NTH in strings reductions and eliminate STRING_TO_CODE (#8851)

Use STRING_NTH in strings reductions and eliminate STRING_TO_CODE (#8851)

This updates our string reductions for `to_lower`, `to_upper`, `from_int`, `to_int` to use STRING_NTH instead of STRING_TO_CODE applied to STRING_SUBSTR.

It optionally replaces STRING_TO_CODE with STRING_NTH during preprocessing (true by default).

This is work towards efficient support for `to_lower`/`to_upper`.