Redesign the platform so that hardware-specific API is not visible by default

At the moment, the hardware-specific API is visible in the arduino sketches by default.
( ie implementation details are bleeding into the Arduino abstract API).

Therefore users can write code like
```
PORTB &= ~(1 << PB3); // reset bit 3 of PORT B
```
and even
```
digitalWrite(PB1 , HIGH); //This doesnt work
```
This confusion can be eliminated if the hw-specific headers are not included in the sketch by default.
If advanced users really need these defines, they can include the headers explicitly.

Samples:
https://www.instructables.com/id/ATTiny-Port-Manipulation/
` PORTB |= (1 << PB3); //replaces digitalWrite(PB3, HIGH);`
https://github.com/damellis/attiny/issues/127

Start by tracing how Arduino sketches receive the hardware-specific headers and how the abstract API is exposed by default. Check the sketch entry point and header inclusion path; done means hardware-specific names such as PORTB and PB3 are no longer available by default, while advanced users can still include the headers explicitly.

Written by the indexing model from the issue text.