Add tinyfsm, start converting core functions to an FSM-based event loop

lib/tinyfsm/examples/elevator/README.md

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+Elevator Project
+================
+
+Example implementation of a simplified elevator logic, using [TinyFSM].
+
+  [TinyFSM]: https://digint.ch/tinyfsm/
+
+
+Overview
+--------
+
+Imagine a elevator having:
+
+ - "Call" button on each floor,
+ - "Floor Sensor" on each floor, triggering an event as soon as the
+   elevator arrives there,
+ - "Alarm" button.
+
+
+Implementation
+--------------
+
+The elevator example implements two state machines interacting with
+each other:
+
+ 1. Elevator
+    - State: Idle
+    - State: Moving
+    - State: Panic
+
+ 2. Motor
+    - State: Stopped
+    - State: Up
+    - State: Down
+
+
+A good state machine design avoids circular dependencies at all
+cost: While the elevator sends events to the motor, the motor NEVER
+sends events to the elevator (top-down only).
+
+
+FAQ
+---
+
+Did you notice the motor starting twice? This is by design, let's
+have a look at the call stack of fsm_list::start() in main.cpp:
+
+    FsmList<Motor, Elevator>::start()
+       Motor::set_initial_state()
+          Motor::current_state = Stopped
+       Elevator::set_initial_state()
+          Elevator::current_state = Idle
+       Motor::enter()
+          Motor:Stopped->entry()
+             cout << "Motor: stopped"               <-- HERE
+             Motor::direction = 0
+       Elevator::enter()
+          Elevator:Idle->entry()
+             send_event(MotorStop)
+                Motor::react(MotorStop)
+                   Motor:Stopped->transit<Stopped>
+                      Motor:Stopped->exit()
+                      Motor::current_state = Stopped
+                      Motor:Stopped->entry()
+                         cout << "Motor: stopped"   <-- HERE
+                         Motor::direction = 0
+                Elevator::react(MotorStop)
+
+If we really had to work around this, we could either:
+
+ 1. Change the initialization (bad design practice!) in main.cpp:
+
+        - fsm_list::start();
+        + fsm_list::set_initial_state();
+        + Elevator::enter();
+
+
+ 2. Modify the Motor:Stopped->entry() function in motor.cpp:
+
+          class Stopped : public Motor {
+            void entry() override {
+        +     if(direction == 0)
+        +       return;
+              cout << "Motor: stopped" << endl;
+              direction = 0;
+            };