Warn that return types cannot be checked for not annotated arguments

[cdsousa]

Hi, great package! A step further in type safety :)

I think that returning nothing in case a function is type safe as well as when the function has a non-annotated argument can be a little ambiguous.
For example, when one doesn't know/remember whether some function is type annotated.

Maybe a warning can be issued for each non-checkable function. And maybe this can be controlled with a optional parameter...