Modification of ArduinoCore-avr to use API

[giulcioffi]

The following proposed modifications are intended to port the API to the AVR boards:

1. rename the "HardwareSerial" classes (from the API) with "UART" (from ArduinoCore-avr);
2. remove functions/definitions from several files in ArduinoCore-avr, since they are already implemented/defined by the API.
   Files modified:
   • "Arduino.h" : it now includes "api/ArduinoAPI.h" and "api/Common.h"
   • "WMath.cpp": few functionalities are already implemented by "api/Common*"
     Files deleted, because included in the API:
   • "Client.h"
   • "IPAddress*"
   • "Print"
   • "Server.h"
   • "Stream*"
   • "Udp.h"
   • "WCharacter.h"
   • "WString*"
   • "binary.h"
3. port USB, SPI and Wire into API: for all of them types u8/u32 have been replaced with standard types;
4. the digital* APIs have been adapted
5. modification of compile recipes (in "platform.txt") to add the API folder.