fn step<AgentRef: Agent, RegAccess: RegisterAccess<AgentRef::Register>>(agent_cast: fn(&mut AgentRef) -> &mut Self, register_cast: fn(&mut AgentRef) -> &mut RegAccess, agent_ref: &Rc<RefCell<AgentRef>>, host_ref: &Rc<RefCell<Host<AgentRef>>>) -> Result<(), StepError> where AgentRef::Register: From<Self::Register>