Support for annotated_types

[JelleZijlstra]

This adds support for checking annotated_types annotations against literals, as well as a few tricks to make them more usable:

• Allowing stronger annotations where a weaker one is requested; for example, allow Annotated[int, Gt(6)] where Annotated[int, Gt(5)] is requested.
• Inferring annotations in some cases (if x > 5: essentially adds the annotation Gt(5)).

Future work:

• We should also infer predicates for if len(x) > 5: but we currently don't.