Callback updates

* Optimise clearing by adding `nullptr` overload. This overload means
  `Callback(NULL)` or `Callback(0)` will no longer work; users must
  use `Callback(nullptr)` or `Callback()`.
* Optimise clearing by not clearing storage - increases code size of
  comparison, but that is extremely rare.
* Reduce ROM used by trivial functors - share copy/destroy code.
* Config option to force trivial functors - major ROM saving by
  eliminating the "operations" table.
* Config option to eliminate comparison altogether - minor ROM saving by
  eliminating zero padding.
* Conform more to `std::function` API.

Remove move stuff

More callback work

Author: kjbracey

features/netsocket/NetworkInterface.cpp

@@ -151,6 +151,7 @@ void NetworkInterface::add_event_listener(mbed::Callback<void(nsapi_event_t, int
     attach(mbed::callback(&call_all_event_listeners, this));
 }
 
+#if MBED_CONF_PLATFORM_CALLBACK_COMPARABLE
 void NetworkInterface::remove_event_listener(mbed::Callback<void(nsapi_event_t, intptr_t)> status_cb)
 {
     iface_eventlist_t *event_list = get_interface_event_list_head();
@@ -162,6 +163,7 @@ void NetworkInterface::remove_event_listener(mbed::Callback<void(nsapi_event_t,
         }
     }
 }
+#endif
 
 NetworkInterface::~NetworkInterface()
 {

features/netsocket/NetworkInterface.h

@@ -319,13 +319,15 @@ class NetworkInterface: public DNS {
      */
     void add_event_listener(mbed::Callback<void(nsapi_event_t, intptr_t)> status_cb);
 
+#if MBED_CONF_PLATFORM_CALLBACK_COMPARABLE
     /** Remove event listener from interface.
      *
      * Remove previously added callback from the handler list.
      *
      *  @param status_cb The callback to unregister.
      */
     void remove_event_listener(mbed::Callback<void(nsapi_event_t, intptr_t)> status_cb);
+#endif
 
     /** Get the connection status.
      *