move debugging methods into private section

Reduces the 'surface area' of the MMU class
connect peripherals to mmu

ugly, but handles many data types for specifying addresses.
connect gpu to mmu
use internal storage for gpu memory instead of relying on mmu
forward writes from mmu to gpu based on address ranges
remove gpu specifics from mmu
store bios separately instead of overwriting main memory
treat cartridge as a peripheral
check bios before peripherals.

preserve expected operational semantics
Forward memory requests to peripherals (gpu, cartridge, etc.) based on address.