API impact:
- install_ram/rom/writeonly now requires a non-null pointer.  If you want
  automatically managed ram, add it to a memory map, not in machine_start

- install_*_bank now requires a memory_bank *, not a string

- one can create memory banks outside of memory maps with memory_bank_creator

- one can create memory shares outside of memory maps with memory_share_creator

Memory maps impact:
- ram ranges with overlapping addresses are not shared anymore.  Use .share()
- ram ranges touching each other are not merged anymore.  Stay in your range

Extra note:
- there is no need to create a bank just to dynamically map some memory/rom.
  Just use install_rom/ram/writeonly