This PR adopts static type hints as per `mypy` recommendations. This
will hopefully clean up the code a little and squash a few type-related
bugs.
Additionally, this will hopefully open up the door to compiling the
project using `mypyc`, which may score us some performance gains without
needing to rewrite the project in an entirely new language.
This branch was originally used to test out Pydantic. However, after
some profiling, I came to the conclusion that runtime type checking is
overkill for a project like this. So, instead, I'm sticking with static
typing via dataclasses.
Fixes#56.
