Returning implicit none can break type signature

[acbart]

Functions don't have to have an explicit return statement. In that case, they return None. If a signature defines that the function returns something else, like a string, then this is incorrect. However, TIFA currently will allow this in the case where there was at least one other return statement in the function that returned the right type. We need to check if the current path has a return statement, and if so, then check that. We cannot rely on there just being a return anywhere in the body. This might be a matter of checking that the return was DEFINITELY set, not MAYBE set.