Buddy allocator should support adding RAM post-initialization

[hawkw]

It would be nice if we could add regions of RAM to the heap after initialising it, rather than using a fixed heap region. That way, we could just go through the Multiboot ELF sections and just snarf every unused section into the heap, rather than having HEAP_BEGIN and HEAP_END pointers in ASM. I've seen this in other people's heap implementations and it shouldn't be that hard to implement.

[hawkw]

Depends on #79.

[hawkw]

This is sort of a less good version of having the heap allocator request frames from the frame allocator, I think.