Buffer/Iterators Improvements

[caemor]

• Should more assertions about slice sizes be made or should it just continue to panic (current status) if someone uses a buffer which is too small (see graphics.rs#L35)?
• Use iterator-based SPI when Update changelog for v0.2.2 release and tick version rust-embedded/embedded-hal#107 v0.2.2 lands and is supported in various hal device crates
• Test and use Add Dimensions trait embedded-graphics/embedded-graphics#61 - dimensions for partial display updates

For the two last points:

Current situation:

1. iterator-based drawing - through embedded-graphics lib
2. fills "randomly" positioned pixel in bigger buffer (u8-slice) - this happens in Drawable::draw()
3. Transmit full buffer via spi

What could be changed and improved:

1. nothing
   2.1 Fill full byte of 8 pixels (u8) and transmit the iterator of these bytes? questionable
   2.2 Use Dimensions traits to only transmit the necessary parts to limit the changed parts and only draw these with update_partial_frame
2. transmit iterator via spi (this would make the linux chunk size limit unecessary)