Do a PPU and logging cleanup pass.

 - LOG_INSTRUCTIONS decodes addresses now, so LOG_PPU_REG is
   no longer needed. Ditto for LOG_CONTROLLER.

 - LOG_PPU_MEM is more clutter than it's worth - remove.

 - LOG_SPRITE_ZERO went unused after sprite zero hit detection
   was rolled into pixel output - remove.

 - Rename read/write_ppu() to read/write_vram() and give them internal
   linkage (oversight).

 - Compactify some switches.

 - Add more comments.