Overhaul memory allocation model

This commit makes several fundamental changes to memory handling:

- the frame allocator is now only an allocator for free frames, and does
  not track used frames.
- the frame allocator now stores its free list inside the free frames
  themselves, as a hybrid stack/span model.
  - This has the implication that all frames must currently fit within
    the offset area.
- kutil has a new allocator interface, which is the only allowed way for
  any code outside of src/kernel to allocate. Code under src/kernel
  _may_ use new/delete, but should prefer the allocator interface.
- the heap manager has become heap_allocator, which is merely an
  implementation of kutil::allocator which doles out sections of a given
  address range.
- the heap manager now only writes block headers when necessary,
  avoiding page faults until they're actually needed
- page_manager now has a page fault handler, which checks with the
  address_manager to see if the address is known, and provides a frame
  mapping if it is, allowing heap manager to work with its entire
  address size from the start. (Currently 32GiB.)

src/include/kernel_memory.h

@@ -22,6 +22,9 @@ namespace memory {
 	/// Initial process thread's stack size, in pages
 	static const unsigned initial_stack_pages = 1;
 
+	/// Max size of the kernel heap
+	static const size_t kernel_max_heap = 0x800000000; // 32GiB
+
 	/// Helper to determine if a physical address can be accessed
 	/// through the page_offset area.
 	inline bool page_mappable(uintptr_t a) { return (a & page_offset) == 0; }