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.

IRremote.cpp

@@ -18,16 +18,17 @@
 // Whynter A/C ARC-110WD added by Francesco Meschia
 //******************************************************************************
 
-#ifndef ESP32
-#include <avr/interrupt.h>
-#endif
-
 // Defining IR_GLOBAL here allows us to declare the instantiation of global variables
 #define IR_GLOBAL
 #	include "IRremote.h"
 #	include "IRremoteInt.h"
 #undef IR_GLOBAL
 
+#ifndef IR_TIMER_USE_ESP32
+#include <avr/interrupt.h>
+#endif
+
+
 //+=============================================================================
 // The match functions were (apparently) originally MACROs to improve code speed
 //   (although this would have bloated the code) hence the names being CAPS
@@ -122,7 +123,7 @@ int  MATCH_SPACE (int measured_ticks,  int desired_us)
 // As soon as first MARK arrives:
 //   Gap width is recorded; Ready is cleared; New logging starts
 //
-#ifdef ESP32
+#ifdef IR_TIMER_USE_ESP32
 void IRTimer()
 #else
 ISR (TIMER_INTR_NAME)