Tiktoken

- Add `tiktoken.encoding_for_model` to get the encoding for a specific model
- Improve portability of caching logic

Thank you to fritzo, arvid220u, khanhvu207, henriktorget for various small corrections

- Avoid use of `blobfile` for public files
- Add support for Python 3.8
- Add py.typed
- Improve the public tests

- Initial release