namcos1.c: Completely rewrote the emulation of the CUS117 MMU.
The MMU is no longer emulated by modifying address maps at
runtime, an operation which has become excessively slow as the
core memory system has accumulated features and complexity over
the years. Instead, the MMU is now a device which works on
similar principles to the address_map_bank_device. The recent
multisession regression is fixed, all games in the driver
should now run substantially faster, and most importantly the
driver code is now much cleaner and easier to understand. [Alex Jackson]