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/kernel/frame_allocator.h

@@ -0,0 +1,70 @@
+#pragma once
+/// \file frame_allocator.h
+/// Allocator for physical memory frames
+
+#include <stdint.h>
+
+#include "kutil/allocator.h"
+#include "kutil/linked_list.h"
+
+struct frame_block;
+using frame_block_list = kutil::linked_list<frame_block>;
+
+/// Allocator for physical memory frames
+class frame_allocator
+{
+public:
+	/// Default constructor
+	frame_allocator();
+
+	/// Get free frames from the free list. Only frames from the first free block
+	/// are returned, so the number may be less than requested, but they will
+	/// be contiguous.
+	/// \arg count    The maximum number of frames to get
+	/// \arg address  [out] The physical address of the first frame
+	/// \returns      The number of frames retrieved
+	size_t allocate(size_t count, uintptr_t *address);
+
+	/// Free previously allocated frames.
+	/// \arg address  The physical address of the first frame to free
+	/// \arg count    The number of frames to be freed
+	void free(uintptr_t address, size_t count);
+
+	/// Get a memory allocator that allocates raw pages
+	/// \returns  The allocator ojbect
+	kutil::allocator & raw_allocator() { return m_raw_alloc; }
+
+private:
+	class raw_alloc :
+		public kutil::allocator
+	{
+	public:
+		raw_alloc(frame_allocator &fa);
+		virtual void * allocate(size_t size) override;
+		virtual void free(void *p) override;
+	private:
+		frame_allocator &m_fa;
+	};
+
+	raw_alloc m_raw_alloc;
+	frame_block_list m_free; ///< Free frames list
+
+	frame_allocator(const frame_allocator &) = delete;
+};
+
+
+/// A block of contiguous frames. Each `frame_block` represents contiguous
+/// physical frames with the same attributes.
+struct frame_block
+{
+	uintptr_t address;
+	uint32_t count;
+
+	/// Compare two blocks by address.
+	/// \arg rhs   The right-hand comparator
+	/// \returns   <0 if this is sorts earlier, >0 if this sorts later, 0 for equal
+	int compare(const frame_block *rhs) const;
+};
+
+
+extern frame_allocator g_frame_allocator;