Implement CPU

[taylus]

• Implement GBZ80 instruction set
  • With a unit test suite covering each instruction in isolation
  • References:
    • https://rednex.github.io/rgbds/gbz80.7.html
    • https://www.pastraiser.com/cpu/gameboy/gameboy_opcodes.html
    • http://imrannazar.com/GameBoy-Emulation-in-JavaScript:-The-CPU