add type hints

This PR adds type hints to the public API of torf. I've tried my best to get the types right with the help of documentation and plain old testing. For this PR, I've chosen to seperate the hints from the implementation but if you would prefer it be moved to implementation, I'm happy to do so. References: - https://mypy.readthedocs.io/en/stable/getting_started.html#dynamic-vs-static-typing - https://fastapi.tiangolo.com/python-types/ - https://docs.python.org/3/library/typing.html - https://typing-extensions.readthedocs.io/en/latest/