Allow page table copying and unmapping

Lots of rearranging in page_manager as well, moving constants out as
well as helper structs.

src/kernel/kernel_memory.h

@@ -0,0 +1,22 @@
+#pragma once
+/// \file kernel_memory.h
+/// Constants related to the kernel's memory layout
+
+namespace memory {
+
+	/// Size of a single page frame.
+	static const size_t frame_size = 0x1000;
+
+	/// Start of kernel memory.
+	static const uintptr_t kernel_offset = 0xffffff0000000000;
+
+	/// Offset from physical where page tables are mapped.
+	static const uintptr_t page_offset = 0xffffff8000000000;
+
+	/// Initial process thread's stack address
+	static const uintptr_t initial_stack = 0x0000800000000000;
+
+	/// Initial process thread's stack size, in pages
+	static const unsigned initial_stack_pages = 1;
+
+} // namespace memory