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/libraries/initrd/include/initrd/initrd.h

@@ -49,7 +49,7 @@ class disk
 public:
 	/// Constructor.
 	/// \arg start  The start of the initrd in memory
-	disk(const void *start);
+	disk(const void *start, kutil::allocator &alloc);
 
 	/// Get the vector of files on the disk
 	const kutil::vector<file> & files() const { return m_files; }

src/libraries/initrd/initrd.cpp

@@ -23,7 +23,8 @@ file::executable() const {
 }
 
 
-disk::disk(const void *start)
+disk::disk(const void *start, kutil::allocator &alloc) :
+	m_files(alloc)
 {
 	auto *header = reinterpret_cast<const disk_header *>(start);
 	size_t length = header->length;