o Remove Cycles chan from CPU/PPU and reorganize PPU's Run/Execute
  methods so that new NES.RunProcessors method can be CPU/PPU in
  lock-step by simplying calling Execute on each, taking into account
  that the PPU gets 3 cycles for each 1 CPU cycle.  This significantly
  speeds up the emulator compared to having the CPU/PPU each executing
  in their own goroutine using channels to pass execution back and forth
  between them.

o Fixed up unit tests to pass again.