Missing SIO disk driver

[ytmytm]

Needs these parts to be implemented:

• InitForIO and DoneForIO to setup/restore anything required by OS ROM to make SIO work (pages $02 and $03 can be freely used, zero page may have to be restored)
• need to read 256 sector at a time using track & sector designation
• there can be up to 255 tracks (track 0 doesn't exist) and up to 256 sectors on each track
• track 18 is special, needs to hold disk header in (18,0) and start of directory in (18,1)
• GEOS will put a Commodore DOS-like filesystem upon that, so no compatibility with native Atari disks
• SetDevice needs to be implemented to swap disk driver from $9000-$9D80 in main RAM with $5000-$5D80 in extended RAM bank 0

SIO can be handled through ROM functions (like cc65 does?) or directly.

[ytmytm]

This seems to implement SIO access without resorting to ROM: https://www.horus.com/~hias/atari/#hipatch