Adding digitalToggle to core?

### Description
The Arduino core has a digitalWrite() and a digitalRead() function.
In many sketches there is a need to toggle a pin, either for a blinking LED or a clock pin for swSPI etc.

There are two typical ways to invert a pin - see code below
```cpp
// using a state holding the value of the pin
digitalWrite(pin, state);
state = 1 - state;

// read pin, invert and write back
digitalWrite(pin, !digitalRead(pin));
```

The latter one is slower as it redo a lot of "pin math", so I implemented a version of **digitalToggle()** for AVR.
Attached test sketch shows the gain compared to the two methods.

### Results test on UNO
```
Time us 1000 calls
Reference: 7392 // read invert write
Var: 4784 // use state var
   Toggle: 3964 // use digitalToggle returning state
   Toggle: 3520 // use digitalToggle returning NO state
```

The gain of toggle returning state is 46% resp 16%
The gain of toggle returning NO state is 52% resp 26%

Imho these gains are interesting, esp for clocking data

### Implementation for AVR

**Arduino.h**
```cpp
uint8_t digitalToggle(uint8_t pin); // returns the new state of the pin
```

**wiring_digital.c**
```cpp
uint8_t digitalToggle(uint8_t pin)
{
uint8_t port = digitalPinToPort(pin);
volatile uint8_t *out;

if (port == NOT_A_PIN) return 0;
uint8_t timer = digitalPinToTimer(pin);
if (timer != NOT_ON_TIMER) turnOffPWM(timer);

uint8_t bit = digitalPinToBitMask(pin);

out = portOutputRegister(port);
uint8_t oldSREG = SREG;
cli();
*out ^= bit; // invert bit
SREG = oldSREG;
return ((*out & bit) != 0);
}
```
Note: a no state returning version is straightforward given the above code.

### Test sketch

[digitalToggle.zip](https://github.com/arduino/Arduino/files/5646909/digitalToggle.zip)

```cpp
uint32_t start, Tref, Tref2, Tnew;
const uint8_t pin = 13;

uint8_t state = LOW;

void setup()
{
Serial.begin(115200);
Serial.println();
Serial.println(__FILE__);

pinMode(pin, OUTPUT);
digitalWrite(pin, LOW);

start = micros();
for (int i = 0; i < 1000; i++) digitalWrite(pin, !digitalRead(pin));
Tref = micros() - start;

start = micros();
for (int i = 0; i < 1000; i++)
{
digitalWrite(pin, state);
state = 1 - state;
}
Tref2 = micros() - start;

start = micros();
for (int i = 0; i < 1000; i++) digitalToggle(pin);
Tnew = micros() - start;

Serial.print("Reference:\t");
Serial.println(Tref);
Serial.print(" Var:\t");
Serial.println(Tref2);
Serial.print(" Toggle:\t");
Serial.println(Tnew);
Serial.print(" Gain:\t");
Serial.println(Tref - Tnew);
Serial.print(" Perc:\t");
Serial.println(100.0 - (100.0 * Tnew) / Tref, 1);

pinMode(13, OUTPUT);
}

void loop()
{
static int cnt = 0;
if (cnt == 60)
{
cnt = 0;
Serial.println();
}
cnt++;
// digitalToggle(pin);
int x = digitalToggle(pin);
Serial.print(x);

delay(1000);
}
```

### Additional context

#### Additional requests

- https://github.com/arduino/ArduinoCore-API/issues/77#issue-494269676
- https://github.com/arduino/ArduinoCore-API/issues/77#issuecomment-287657867
- https://github.com/arduino/ArduinoCore-API/issues/77#issuecomment-287658811
- https://github.com/arduino/ArduinoCore-API/issues/77#issuecomment-287660708
- https://github.com/arduino/ArduinoCore-API/issues/77#issuecomment-287696536
- https://github.com/arduino/ArduinoCore-API/issues/77#issuecomment-287732174
- https://github.com/arduino/Arduino/pull/111
- https://github.com/arduino/Arduino/issues/11777

Beginne damit, die vorgeschlagene Arduino.h-Deklaration und die AVR-Implementierung in wiring_digital.c zu lesen, und prüfe anschließend ArduinoCore-API issue 77 sowie die verlinkten Arduino issues und den pull request. Als abgeschlossen gilt die Aufgabe, wenn die API-Form, einschließlich des Rückgabewerts, festgelegt wurde und bestimmt wurde, ob der Vorschlag über die gezeigte AVR-Implementierung hinaus geeignet ist.

Vom Indexierungsmodell aus dem Issue-Text verfasst.