External Interrupt handling fix

[cmaglie]

• NMI Interrupts are now correctly ignored (no more hardcoded pin numbers)
• callbacksInt doesn't need to be a structure, changed to a simple array of function pointers.

This pull request together with 0368e10 fix issue #30.

/cc @aethaniel @facchinm @manitou48

[ArduinoBot]

✅ Build completed.

⬇️ Build URL: http://downloads.arduino.cc/PR/samd/package_samd-b27_index.json

ℹ️ To test this build:

1. Open the Preferences of the Arduino IDE.
2. Add the Build URL above in the Additional Boards Manager URLs field, and click OK.
3. Open the Boards Manager (menu Tools->Board->Board Manager...)
4. Install Arduino SAMD core - Pull Request External Interrupt handling fix #32
5. Select one of the boards under SAMD Pull Request External Interrupt handling fix #32 in Tools->Board menu
6. Compile/Upload as usual

[aethaniel]

Seems 🆗 to me

[facchinm]

👍