Cleaned up ESP32 integration, reverted ESP32 ifdefs on irreceive examples.

- fixed indenting on existing code in a few places for consistency
- introduced IR_TIMER_USE_ESP32 for ifdefs within the code as per
  request
- added comments explaining what's missing for irsend support on ESP32
- IRrecvDemo.ino gets a warning before and after interrupt is enabled in
  case it causes a crash

TESTED=IoTuz ESP32 board and original 328p arduino to make sure current
code did not break.

examples/IRrecvDump/IRrecvDump.ino

@@ -15,11 +15,7 @@
 *  You can change this to another available Arduino Pin.
 *  Your IR receiver should be connected to the pin defined here
 */
-#ifdef ESP32
-int RECV_PIN = 35;
-#else
 int RECV_PIN = 11;
-#endif
 
 IRrecv irrecv(RECV_PIN);