first cut at implementing 64KB shadow RAM to eliminate wait states
from dual-port chip RAM.
tell ISE project about shadowram.vhdl
excise reading from chipram in favour of always shadowing all 128KB
of chipram.
expand shadowram to 256KB, mapped in full at a configurable location.
fix default location and decoding of shadow RAM.
tighten up state machine control when reading from shadow ram.
fix reset handling during simulation.
adjust simulation debug output a bit.
fix bug with writing to shadow ram.
remove test instruction from kickstart rom entry.