0f47810768
This mostly only improves things for incorrect arguments and for an incorrect return type. It doesn't do much to improve the case where `__bool__` isn't callable and leaves the union/other cases untouched completely. I picked this one because, at first glance, this _looked_ like a lower hanging fruit. The conceptual improvement here is pretty straight-forward: add annotations for relevant data. But it took me a bit to figure out how to connect all of the pieces.
Markdown files within the mdtest/ subdirectory are tests of type inference and type checking;
executed by the tests/mdtest.rs integration test.
See crates/red_knot_test/README.md for documentation of this test format.