leaky page frame allocator

[hawkw]

The kernel will need a simple page frame allocator to start allocating the initial frames that we'll use permanently, before we set up a page frame allocator with bookkeeping. It's okay for this allocator to leak frames, as we'll never want to de-allocate these initial frame allocations for e.g. re-mapping the kernel.

It seems to me that the easiest and possibly the most compose-able way to write this allocator is just to implement FrameAllocator for Iterator<Item=Frame>. The various architecture-specific memory map providers in the kernel can all return iterators over memory areas, and we can map them into a frame representation and then use that iterator as an allocator.

it should:

• on calls to allocate(), call iter.next() and return Ok(frame) if next() returns Some or Err(AllocErr::Exhausted) if iter.next() returns None.
• on calls to deallocate(), panic. (or silently do nothing; but i think if we ever try to deallocate these frames that's worthy of a kernel oops).
• if/when we add an allocate_range() type API, return AllocErr::Unsupported on all calls to that function

[hawkw]

This is probably not terribly hard but does require some background knowledge.