[kernel] Begin replacing page_manager with vm_space

This is the first commit of several reworking the VM system. The main
focus is replacing page_manager's global functionality with objects
representing individual VM spaces. The main changes in this commit were:

- Adding the (as yet unused) vm_area object, which will be the main
  point of control for programs to allocate or share memory.
- Replace the old vm_space with a new one based on state in its page
  tables. They will also be containers for vm_areas.
- vm_space takes over from page_manager as the page fault handler
- Commented out the page walking in memory_bootstrap; I'll probably need
  to recreate this functionality, but it was broken as it was.
- Split out the page_table.h implementations from page_manager.cpp into
  the new page_table.cpp, updated it, and added page_table::iterator as
  well.

src/kernel/page_manager.h

@@ -118,11 +118,6 @@ public:
 	/// Get a pointer to the kernel's PML4
 	inline page_table * get_kernel_pml4() { return m_kernel_pml4; }
 
-	/// Attempt to handle a page fault.
-	/// \arg addr  Address that triggered the fault
-	/// \returns   True if the fault was handled
-	bool fault_handler(uintptr_t addr);
-
 private:
 	/// Copy a physical page
 	/// \arg orig  Physical address of the page to copy
@@ -182,6 +177,7 @@ private:
 	frame_allocator &m_frames;
 
 	friend class memory_bootstrap;
+	friend class vm_space;
 	page_manager(const page_manager &) = delete;
 };