fix(core): Add missing 'FSM_NAME' associated constant

This one got lost while moving from the prototype code to the proper
library.

finito-door/src/lib.rs

@@ -40,6 +40,7 @@ pub enum DoorAction {
 }
 
 impl FSM for DoorState {
+    const FSM_NAME: &'static str = "door";
     type Event = DoorEvent;
     type Action = DoorAction;