begin implementing MAP instruction.  It should work in a crude way,
but doesn't take ROM/IO into account.  C65GS memory map probably
needs to shadow ROMs into $0020000 - $003FFFF to match C65 memory
map to make this all a bit easier.  Issue #25.
push CPU back down to 64MHz to meet timing until we have time to
reoptimise timing after adding in 4510 instructions.  Issue #25