Generic screen traits & structs, untested RPi impl

kernel/screen.rs is a new module for displays (screens). Some basic
traits for pixels, the screen itself, color depth, and resolutions.
Builtin constants for common display resolutions & color depths. Should
think about adding some additional IO traits that all devices adhere to.

arch/arm/drivers/arm1176jzf_sr.rs is a new module for Raspberry Pi
support. I've ported some C code for dealing with the GPU, though it's
as yet incomplete (don't bother trying to run it). Will continue to
update & test; first will actually be the (now-empty) GPIO section.

Dunno if there's a good way to do hardware discovery on startup, or
whether it's separate builds. This *shouldn't* mess with the qemu path,
but I've not got that running. Will be sure to test before pushing back
to wbthomason/ironkernel.