feat: implement lazy page allocation

[JonasKruckenberg]

Right now we eagerly allocate physical memory to back all allocations which isn't great (but it's simple 🙃). Instead we should implement a lazy allocation scheme where physical memory is reserved but not only comitted when first accessed (by catching page faults).

We essentially want 3 states a page can be in:

• Free - The page is unused and inaccessible, accessing it will result in a kernel panic.
• Reserved - The page is reserved for future use and cannot be assigned other programs. With lazy committing enabled, accessing it will result in a page fault that causes the page to be comitted.
• Committed - A physical frame has been allocated to back this page. The page is fully accessible (subject to the memory protection flags).

Reserved pages should be able to be committed on-demand using methods and we should be able to reserve & commit memory in one operation too.