Require strict `delegate=` subtyping

Right now - any type can be a delegate. We don't check anything, unless it is used as instance annotation.
But, it leads to potential errors. One can use different `delegate` and `instance` types.

It would be hard to debug, why your instance handler  is not called / crashes in runtime.

So, the solution is to require `delegate=` to be a strict subtype of `instance` type.