Add type annotations

[RobertCraigie]

Type annotations would improve the editor experience and support static type checking. Type annotations would only really have to be added to user facing code but it would still be beneficial to type the whole codebase as well.

I'd be happy to help with this if it is deemed acceptable :)

[LewisGaul]

I think the only thing needed to allow users to annotate their code is to expose the BenchmarkFixture class from __init__.py?

[RobertCraigie]

Potentially. A py.typed file would have to be added as well.