- Made `NativeApi` into a `static class` (it's not intended to be instantiated)
 - Moved `LLamaTokenType` enum out into a separate file
 - Made `LLamaSeqId` and `LLamaPos` into `record struct`, convenient to have equality etc

- Saves with serialized ChatHistory of session
- Only allows use of ChatHistory.Message (instead of raw text)
   for easy post-processing with IHistoryTransform implementation
- Provides History Management methods
- Allows user to regenerate last assistant message