Access to non-available memory areas

[CalebLBaker]

The MemoryMapTag struct in memory_map.rs provides a memory_areas function that allows iterating over memory areas, but it skips memory areas that are any type other than available. It would be helpful to get an iterator over all memory areas. I suggest adding a function all_memory_areas to MemoryMapTag which would provide an iterator that doesn't skip any areas.

[CalebLBaker]

I created a fork of the repository that addresses this issue

https://github.com/CalebLBaker/multiboot2-elf64