Add type hints

[leotrs]

There are many benefits to type hints that we can go over our next call. I've been holding off on proposing type hints until now because the main classes were still in flux.

However, I recently found a bug (#406) that is exactly the kind of bug that can be caught early when using type hints. I'm sure there are others we haven't caught yet.

Migrating the entire library to type hints is laborious but I can take on that challenge.