[boot][kernel] Replace frame allocator with bitmap-based one

The previous frame allocator involved a lot of splitting and merging
linked lists and lost all information about frames while they were
allocated. The new allocator is based on an array of descriptor
structures and a bitmap. Each memory map region of allocatable memory
becomes one or more descriptors, each mapping up to 1GiB of physical
memory. The descriptors implement two levels of a bitmap tree, and have
a pointer into the large contiguous bitmap to track individual pages.

src/include/kernel_memory.h

@@ -35,11 +35,17 @@ namespace memory {
 	constexpr uintptr_t stacks_start = heap_start - kernel_max_stacks;
 
 	/// Max size of kernel buffers area
-	constexpr size_t kernel_max_buffers = 0x10000000000ull; // 1TiB
+	constexpr size_t kernel_max_buffers = 0x8000000000ull; // 512GiB
 
 	/// Start of kernel buffers
 	constexpr uintptr_t buffers_start = stacks_start - kernel_max_buffers;
 
+	/// Max size of kernel bitmap area
+	constexpr size_t kernel_max_bitmap = 0x8000000000ull; // 512GiB
+
+	/// Start of kernel bitmap
+	constexpr uintptr_t bitmap_start = buffers_start - kernel_max_bitmap;
+
 	/// First kernel space PML4 entry
 	constexpr unsigned pml4e_kernel = 256;