Add type hints

[MJGaughran]

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. For example, a Protocol is used here to make the list of additional attributes on an AugmentedValue clear.

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