refactor the use of g_APinDescription ?

It bothers me, in a sort of "Code Purity" sense, that so many core and library functions access
the g_APinDescription[] (for sam/samd) or digital_pin_to_xxx[] (for avr) arrays directly.
There are some macros in variant.h or Arduino.h (digitalPinToBitMask and similar), but they are not consistently used, not all functions have macros, and sometimes they aren't well-placed WRT redefining them for new board types.
Example:

```
variants/mkr1000/variant.h:47: #define digitalPinToBitMask(P) (1 << g_APinDescription[P].ulPin)
cores/arduino/Tone.cpp:133: portBitMask = (1ul << g_APinDescription[outputPin].ulPin);
```

The definition of a more formal API presents the opportunity to offer more formal rules:

1. macros or inline functions to access all pin-related data should be defined in the variant-specific files, or perhaps WVariant.h for core-wide data.

1. if such definitions are defined in core-wide functions, it should be possible to override them in variant-specific files.

1. All other code should use these definitions, instead of assuming a particular implementation. (the tone.cpp example above should not exist, even now.)

The immediate practical benefit would be the possibility of more compact implementations for the "tiny" chips (avr tiny, SAMD11, etc), and greater portability of the functions in the "upper level" areas of code.

首先比较 variant.h 和 Arduino.h 中的访问宏与 cores/arduino/Tone.cpp 中的数组直接使用情况，包括 g_APinDescription 和 digital_pin_to_xxx。梳理当前直接访问的引脚相关数据，然后定义可重写正式 API 的范围；当 core 和库代码使用这些定义而不假定特定实现时，即视为完成。

由索引模型根据 Issue 内容生成。