[BUGFIX] move ISR to IRAM

[foertel]

Fixes #7

[CrashOverride85]

Thanks for this 🙂
I've tested it and it works well on an ESP32 (and I assume ESP8266), but it's no longer building for Arduino Uno / AVR as IRAM_ATTR is unknown ("error: ‘IRAM_ATTR’ does not name a type").

I think it could be fixed by adding something like this to collar.h, just after the #include "Arduino.h" line:

#ifndef ARDUINO_ARCH_ESP8266
  #define IRAM_ATTR
#endif

#ifndef ARDUINO_ARCH_ESP32
  #define IRAM_ATTR
#endif