## Summary
As per our naming scheme (at least for callable types) this should
return a `BoundMethodType`, or be renamed, but it makes more sense to
change the return type.
I also ensure `ClassType.into_callable` returns a `Type::Callable` in
the changed branch.
Ideally we could return a `CallableType` from these `into_callable`
functions (and rename to `into_callable_type` but because of unions we
cannot do this.
ef1802b94f