Attempts at shit but nothing serious. Notes:
* frameskipper doesn't skip enough frames when two or more consecutive VBlanks are missed. This is the cause of the slowdown in Bomberman, Super Metroid and other games. (oh well, the real cause is that the emulator is too slow)
* some ideas laid out for a new CPU event scheduler. Would be faster (less naive IRQ check) and more accurate (emulating DMA delays and such).
* messing with thread priorities
* tracking of open bus reads
Remove cpu_sched.s that has never served a purpose.