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.cpp

@@ -16,7 +16,7 @@
 
 using memory::initial_stack;
 
-scheduler scheduler::s_instance(nullptr);
+scheduler scheduler::s_instance(nullptr, kutil::allocator::invalid);
 
 const uint64_t rflags_noint = 0x002;
 const uint64_t rflags_int = 0x202;
@@ -28,9 +28,10 @@ extern "C" {
 
 extern uint64_t idle_stack_end;
 
-scheduler::scheduler(lapic *apic) :
+scheduler::scheduler(lapic *apic, kutil::allocator &alloc) :
 	m_apic(apic),
-	m_next_pid(1)
+	m_next_pid(1),
+	m_process_allocator(alloc)
 {
 	auto *idle = m_process_allocator.pop();
 	uint8_t last_pri = num_priorities - 1;