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/tests/linked_list.cpp

@@ -152,6 +152,7 @@ TEST_CASE( "Sorted list tests", "[containers list]" )
 	std::uniform_int_distribution<int> gen(1, 1000);
 
 	linked_list<sortableT> slist;
+	CHECK( slist.length() == 0 );
 
 	std::vector<list_node<sortableT>> sortables(test_list_size);
 	for (auto &i : sortables) {