Implement memory bus

[taylus]

Memory in the Game Boy isn't just a 2D array of bytes. Certain ranges go to different I/O devices and etc. Can still treat it like a simple array for unit tests, but the emulator proper needs to adhere to the Game Boy memory map.

References:

• http://imrannazar.com/GameBoy-Emulation-in-JavaScript:-Memory
• https://medium.com/@raphaelstaebler/memory-and-memory-mapped-i-o-of-the-gameboy-part-3-of-a-series-37025b40d89b