Added a dynamic memory implementation using mmap

[Nabushika]

Hope I'm not stepping on toes here - if this is planned for a future stream, please feel free to reject the PR. I'm just very excited about the language, and happy that there's useful stuff I can do with it :)

Still todo:

• Python emulation of mmap/munmap syscalls (should be easy to self-host!)
• Better memory allocation - only whole pages right now, would be very expensive for a lot of small allocations - I seem to recall there's a certain YouTube video about writing your own malloc....

640K? Pah!