Ability to change bit order + SPI support

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added SPI support. arduino uno can shift data out at up to 8mhz with almost 0 cpu cycles. also, no breaking changes ;)

[Simsso]

That's pretty cool, thanks for the contribution. (I'll test this before merging.)

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That's pretty cool, thanks for the contribution. (I'll test this before merging.)

ok ;) how long it could take?

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#define SHIFT_REGISTER_USES_SPI_WITH_FREQUENCY 20000000UL

#include <ShiftRegister74HC595.h>
#include <SPI.h>

#define LATCH 7

ShiftRegister74HC595<3> sr(LATCH);

void setup() {
  Serial.begin(115200);
  
  TCCR1A = 0;
  TCCR1B = 0;
  TCNT1 = 0;
  OCR1A = 10; 
  TCCR1B |= (1 << WGM12);
  TCCR1B |= (1 << CS21);
  TIMSK1 |= (1 << OCIE1A);
  sei();
}

volatile uint8_t b[3] = { 0b10010011, 0b11010011, 0b1111000 };

ISR(TIMER1_COMPA_vect)
{
  sr.setAll(b);
}

void loop() { 
  delay(1000);
}

(signal is not clear because of long jumpwires)

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hey, any progress / problems with this PR?

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hey @Simsso what do you think about this? is there anything i can improve?