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/process.cpp

@@ -1,3 +1,4 @@
+#include "kutil/heap_allocator.h"
 #include "cpu.h"
 #include "debug.h"
 #include "log.h"
@@ -5,7 +6,7 @@
 #include "scheduler.h"
 
 extern "C" void task_fork_return_thunk();
-
+extern kutil::heap_allocator g_kernel_heap; // TODO: this is a bad hack to get access to the heap
 
 void
 process::exit(uint32_t code)
@@ -67,7 +68,7 @@ process::setup_kernel_stack()
 	constexpr unsigned null_frame_entries = 2;
 	constexpr size_t null_frame_size = null_frame_entries * sizeof(uint64_t);
 
-	void *stack_bottom = kutil::malloc(initial_stack_size);
+	void *stack_bottom = g_kernel_heap.allocate(initial_stack_size);
 	kutil::memset(stack_bottom, 0, initial_stack_size);
 
 	log::debug(logs::memory, "Created kernel stack at %016lx size 0x%lx",