o Modified MappableMemory interface such that Mappings() returns two
  arrays, one for addresses mapped for fetches and one for stores.

o Update MappableMemory.AddMappings to understand the new Mappings()
  return values.

o Modified APU.Mappings to return new (fetch, store []uint16) format

o Renamed NTSC/PAL_CLOCK_DIVISOR to NTSC/PAL_CPU_CLOCK_DIVISOR