Switch `prerendered` to use the same bit patterns as the normal variant

[david-sawatzke]

This makes the code much simpler, but constrains the available frequencies (but even the AVR spi peripheral has enough prescaler options).

Nice-to-have features, PRs welcome:

• Add an option to use spi::blocking::Write, if it's properly implemented
• If the mosi_idle_high option isn't activated, the flush can be done separately, decreasing the buffer size by 20 bytes

Might be of interest to @rahix, I've added an avr example in https://github.com/smart-leds-rs/smart-leds-samples/pull/7 using avr-hal

[Rahix]

Might be of interest to @Rahix, I've added an avr example in smart-leds-rs/smart-leds-samples#7 using avr-hal

Nice, I've had people ask me whether this works a lot lately. Thanks!