describe why we use no virtual-dtor

Building with `-Wnon-virtual-dtor` prints warnings for each
`tinyfsm::Fsm` class:

> warning: 'class Elevator' has virtual functions and accessible
> non-virtual destructor [-Wnon-virtual-dtor]

Adding a virtual destructor would waste resources (at least one vtable
entry). There is no reason to have a one, as the MooreMachine class
(and derived classes) is NOT intended to be deleted through a pointer
to it.

Building `examples/api/moore_machine.cpp` results in:

- without virtual destructor:

    # size -B -d moore_machine
       text    data     bss     dec     hex filename
       3277     768     584    4629    1215 moore_machine

- with virtual destructor in `MooreMachine`

    # size -B -d moore_machine
       text    data     bss     dec     hex filename
       3662     808     600    5070    13ce moore_machine

Ref: #21

examples/api/Makefile

@@ -35,7 +35,7 @@ CXXFLAGS    += -fno-rtti
 CXXFLAGS    += -Wall -Wextra
 CXXFLAGS    += -Wctor-dtor-privacy
 CXXFLAGS    += -Wcast-align -Wpointer-arith -Wredundant-decls
-CXXFLAGS    += -Wshadow -Wcast-qual -Wcast-align -pedantic
+CXXFLAGS    += -Wshadow -Wcast-qual -Wcast-align -pedantic -Wnon-virtual-dtor
 
 # Produce debugging information (for use with gdb)
 #OPTIMIZER  = -Og

examples/api/moore_machine.cpp

@@ -14,6 +14,10 @@ struct Switch : tinyfsm::MooreMachine<Switch>
 {
   /* pure virtual reaction (override required in all states) */
   virtual void react(Toggle const &) = 0;
+
+//  ~Switch() {
+//    std::cout << "* ~Switch()" << std::endl;
+//  }
 };
 
 

include/tinyfsm.hpp

@@ -224,6 +224,17 @@ namespace tinyfsm
   template<typename F>
   struct MooreMachine : tinyfsm::Fsm<F>
   {
+    // On the one hand giving an object a virtual destructor means it
+    // will have a vtable and therefore consume 4 (or 8 on 64 bit
+    // machines) additional bytes per-object for the vptr.
+    //
+    // For a class not intended to delete through a pointer to it,
+    // there is no reason whatsoever to have a virtual destructor. It
+    // would not only waste resources, but more importantly it would
+    // give users a wrong hint.
+    //
+    //virtual ~MooreMachine() noexcept = default;
+
     virtual void entry(void) { };  /* entry actions in some states */
     void exit(void) { };           /* no exit actions */
   };