REF: do slicing before calling Block.to_native_types
Related upcoming steps:

- Dispatch DatetimeBlock.to_native_types to DatetimeArray._format_native_types (identical behavior
- Dispatch TimeDeltaBlock.to_native_types to TimedeltaArray._format_native_types (behavior changing, but there is a FIXME in the Block version)
- Dispatch FloatBlock.to_native_types to Float64Index._format_native_types
- Possibly rename to_native_types to something more informative?  (i think there's an issue about deprecating/renaming the Index method)
- in io.csvs use `df._data.apply` instead of iterating over blocks.  Still touches internals, but its a lighter touch.