Use typeguard to check types at runtime

[Andy-Grigg]

Closes #155
Closes #159

Use typeguard to perform run-time type-checking of public class constructors. As reported in the associated tickets, if the correct types are not used then the error messages aren't very clear. typeguard helps here.

Downsides of this approach:

• It doesn't feel very Pythonic. We're using type hints, and any decent IDE or mypy will alert you to your mistake before you run anything.
• Potential performance impacts, but they're not likely to be significant compared to the actual async import type. Also, typeguard will disable these checks if you run Python in 'production mode'.
• It's inconsistent. We're not going to add it anywhere, so why just here?