Switch page_manager to use frame_allocator.

Removed the frame allocation logic from page_manager and replaced it
with using an instance of frame_allocator instead. This had several
major ripple effects:

- memory_initalize() had to change to support this new world
  - Where to map used blocks is now passed as a flag, since blocks don't
    track their virtual address anymore
  - Instead of the complicated "find N contiguous pages that can be
    mapped in with one page table", we now just have the bootloader give
    us some (currently 64) pages to use both for tables and scratch
    space.
  - frame_allocator initialization was split into two steps to allow
    mapping used blocks before std::move()ing them over

src/include/kernel_data.h

@@ -13,7 +13,8 @@ struct popcorn_data {
 	uint16_t version;
 	uint16_t length;
 
-	uint32_t _reserved0;
+	uint16_t _reserved0;
+	uint16_t scratch_pages;
 	uint32_t flags;
 
 	void *initrd;