MemoryReader::read8 reads the CPU bus, where banks 1 and up are whichever bank
the cartridge last switched to, so a table in another bank could only be read by
writing the mapper register -- and the joypad is the only write this workspace
makes into a running game. read_rom takes a bank and a CPU address and reads the
cartridge image the process already holds instead. Defaulted to None, so every
reader without a cartridge behind it narrows rather than guesses.
