[Rust] [DataFusion] Review the contract between DataFusion and Arrow

I am creating this issue based on the discussion at the sync call earlier today.

Apparently DataFusion is not only using the high-level Arrow API but is also accessing Arrow internals directly and this would be one challenge in moving to a majorly refactored Arrow implementation.

Perhaps we need to review what the public Arrow API should be and which APIs DataFusion should or should not be using.