debugger/symbols.go: handle newer debug format.

Should continue to handle old format; currently untested.
go6502.go: revised memory map due to pda6502 V1 schematic error.

74CH138 inputs reversed; splits ROM into two, moves VIA.
SD card on VIA port B pins 0,4,6,7.

This matches the micro SD slot on adafruit's 240x320 TFT ILI9340 board,
as connected to the port B connector on pda6502 V1 hardware.
sd: sdStateCode type with generated stringer.
sd: SdCard renamed to SdCardPeripheral.

This is the type which meets the via6522.ParallelPeripheral interface,
but doesn't implement the SD card details.
sd: overhaul type names; SdCardPeripheral, sdCard, state.
sd: response type with stringer, instead of uint8.