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/scheduler.h

@@ -3,6 +3,7 @@
 /// The task scheduler and related definitions
 
 #include <stdint.h>
+#include "kutil/allocator.h"
 #include "kutil/slab_allocator.h"
 #include "process.h"
 
@@ -30,7 +31,8 @@ public:
 
 	/// Constructor.
 	/// \arg apic  Pointer to the local APIC object
-	scheduler(lapic *apic);
+	/// \arg alloc Allocator to use for TCBs
+	scheduler(lapic *apic, kutil::allocator &alloc);
 
 	/// Create a new process from a program image in memory.
 	/// \arg name  Name of the program image