Add lots of type hints, fix some bugs, and clean up some code

I may have gone a bit overboard. See individual commits.

pyproject.toml

@@ -53,3 +53,4 @@ addopts = [
 [tool.mypy]
 check_untyped_defs = true
 disallow_incomplete_defs = true
+disallow_untyped_decorators = true