This commit refactors the ioregs:
* Use bitfield crate to implement the GPU ioregs.
* IoRegs are stored in their own variables bindings (i.e, Gpu related ioregs are now fields of the Gpu struct)
- This optimize performance quiet alot from my testings - since every scanline was accessing deseralizing ioregs from sysbus. (Getting constant 59fps now)
* For now, comment out DMA model
Also, cleaned the code up to eliminate rustc warnings.
Former-commit-id: 9077695c446ebd1a71783acfdd9819245aa02d7a
