[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/buffer_cache.cpp

@@ -1,7 +1,8 @@
 #include "kutil/assert.h"
-#include "kernel_memory.h"
-#include "page_manager.h"
 #include "buffer_cache.h"
+#include "kernel_memory.h"
+#include "objects/vm_area.h"
+#include "page_manager.h"
 #include "vm_space.h"
 
 extern vm_space g_kernel_space;
@@ -33,16 +34,17 @@ buffer_cache::get_buffer()
 		m_next += m_size;
 	}
 
-	g_kernel_space.commit(addr, m_size);
+	vm_space &vm = vm_space::kernel_space();
+	vm.allow(addr, m_size, true);
+
 	return addr;
 }
 
 void
 buffer_cache::return_buffer(uintptr_t addr)
 {
-	void *ptr = reinterpret_cast<void*>(addr);
-	size_t page_count = page_manager::page_count(m_size);
-	page_manager::get()->unmap_pages(ptr, page_count);
-	g_kernel_space.unreserve(addr, m_size);
+	vm_space &vm = vm_space::kernel_space();
+	vm.allow(addr, m_size, false);
+
 	m_cache.append(addr);
 }