Change digitalPinToInterrupt to return pin, and use g_APinDescription[pin].ulExtInt to map pin inside attachInterrupt/detachInterrupt

This brings things inline with the atachInterrupt documentation for
usage:
attachInterrupt(digitalPinToInterrupt(pin), ISR, mode);

Port of @tico-tico’s change in
https://github.com/tico-tico/Arduino/commit/a7454b6b5c59187b95c4224aad87
bb01faa06e85 to SAMD core.

This prevents accidental overwrites of the bootloader if a NULL
pointer is used for writing by mistake.

Resolve compiler warning from performing arithmetic on void pointers

To be compliant with the one sent by the AVR core

Signed-off-by: Thibaut VIARD <aethaniel@sam-geek.org>

Also add Stream::find(char) API.

Port of: https://github.com/arduino/Arduino/pull/3337

The variant's file compliance is now determined using a define named

ARDUINO_SAMD_VARIANT_COMPLIANCE

that refer to the minimum core version required for this variant file
to work.

The attribute ATTR_ANALOG is needed to select DAC, so it has been
removed from pins A1..A5 that doesn't have a DAC.

The selection of the correct timer peripheral is now
decided through the PIN_ATTR_{TIMER/TIMER_ALT} attribute.

Adjust requestFrom, endTransmission, and startTransmissionWire to handle
repeated starts

See https://github.com/arduino/Arduino/issues/3614

This fixes issue #59 and optimize the way the prescaler is found.
Furthermore non needed instructions are not repeated.

The shortcut has some issues:

- sometimes it fails when sending an odd number of bytes (may be
  due to memory alignment?)
- the data pointer should point to "stable" data (and this is not
  guaranteed by caller, it may be some sort of temporary buffer)
- the SRAM is not guaranteed to start at 0x20000000

All the above problems must be properly fixed before reenabling
this part