[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.

modules.yaml

@@ -42,7 +42,9 @@ modules:
             - src/kernel/objects/kobject.cpp
             - src/kernel/objects/thread.cpp
             - src/kernel/objects/process.cpp
+            - src/kernel/objects/vm_area.cpp
             - src/kernel/page_manager.cpp
+            - src/kernel/page_table.cpp
             - src/kernel/pci.cpp
             - src/kernel/scheduler.cpp
             - src/kernel/screen.cpp