[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/objects/process.cpp

@@ -10,6 +10,7 @@ kutil::vector<process*> process::s_processes;
 process::process(page_table *pml4) :
 	kobject(kobject::type::process),
 	m_pml4(pml4),
+	m_space(pml4),
 	m_next_handle(0),
 	m_state(state::running)
 {