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/IRrecvDemo/IRrecvDemo.ino

@@ -8,11 +8,7 @@
 
 #include <IRremote.h>
 
-#ifdef ESP32
-int RECV_PIN = 35;
-#else
 int RECV_PIN = 11;
-#endif
 
 IRrecv irrecv(RECV_PIN);
 
@@ -20,7 +16,9 @@ decode_results results;
 
 void setup()
 {
-  Serial.begin(115200);
+  Serial.begin(9600);
+  // In case the interrupt driver crashes on setup, give a clue
+  // to the user what's going on.
   Serial.println("Enabling IRin");
   irrecv.enableIRIn(); // Start the receiver
   Serial.println("Enabled IRin");