While I have found some conflicting details on how this should be done,
it seems pretty clear to me that IDX and IDY work with 16-bit addresses,
not 8-bit. The inability to do so was preventing us from probing
peripheral ROM (e.g. at $C705, for the disk controller).
I'm still not confident this is how it _should_ work, but I have found
conflicting accounts online for the behavior of these two modes. The
current code is what allows the Apple II to bootstrap as it should.
As the Apple II (or rather the 6502 chip) is little-endian, meaning the
least significant byte is the first byte you read going forward, rather
than the last byte.
This executes a single opcode. Note this also makes a change to the
get_address_resolver function such that accepts an opcode, not the
address mode itself.
