Use the memory descriptor API.

GBC sure loves bankswitching, and bankswitched address spaces can't be represented by any static mapping system; neither mine, nor any plausible replacement (and anything that can represent bankswitching would be impossible to use if 0xC043 suddenly points to something else).
So let's make use of the address space support I added. Dumping the three plausible memory regions (RAM, SRAM, ROM) in separate address spaces is probably the best we can ask for.