This should save some cycles inside ISR Handler.

Change the way ISRs are stored. Store now in a list on a ‘first come
first served basis’, the first interrupt that gets attached will be
served first. This improves the speed of the ISR calling from about 6us
to 2us for a single interrupt on extint14

See #200

Solve #199

Fix #223

See https://github.com/arduino/Arduino/issues/4525

PWM frequency 732 Hz

The RXC interrupt was being disabled when using bit to disable the DRE
interrupt.

These functions were changed from private to protected but the comments were not updated at that time.

In conjunction with equivalent pull requests to Arduino AVR Boards and Arduino SAM Boards, solves #6146.

Add platform specific keywords.txt

Since https://github.com/arduino/Arduino/commit/3f1b23bf6123e680ed1ddc85d5f39b3555aade0b this will cause keywords in this file to be highlighted only when a board from this platform is selected.

Currently only contains INPUT_PULLDOWN. Style copied from the INPUT_PULLUP entry at https://github.com/arduino/Arduino/blob/1.8.3/build/shared/lib/keywords.txt#L11.

I don't know whether there are other platform specific keywords for Arduino SAMD Boards. I'm happy to add any others on request.

Closes https://github.com/arduino/Arduino/issues/3534