Attiny85 spi driver

[DeepHorizons]

Preliminary SPI driver for the attiny85 using the USI. Ive tested writing data using a logic analyzer. I don't know if I can consume any of the existing settings to set the SPI mode. I have not tested reading and I suspect it would not work [well] because it wont block on a read. I think it would need a modification of the avr-hal-generic::spi::Spi impl but I wanted to get some guidance first.