Add type hints

At the moment, cothread does not play nicely with type hinting in the rest of your code.

This could be done to a limited extent, as with [aioca](https://github.com/dls-controls/aioca/). For example, a `Protocol` is used [here](https://github.com/dls-controls/aioca/blob/master/aioca/types.py#L96) to make the list of additional attributes on an `AugmentedValue` clear.

Alternatively, this could be done more comprehensively using constrained `TypeVar`s and Generics. Python typing is not perfect, so this may introduce problems.