New String/Schema

[ritchie46]

Schema and String type streamlining

• One small string type for all Fields in Arrow and Polars
  This will have a Polars interface. E.g. struct PlSmallStr.
• Polars Schema to polars_schema crate and generic over (Arrow)DataType, where key: PlSmallStr and value generic over DataType

String type requirements:

• O(1) clone
• SmallStr optimization
• Preserving
• Field has constructors new_from_str, new_from_owned, new_from_static

[nameexhaustion]

#18539 (..), #18425 (..)